Our CI suffers from cache thrashing

At this point in time, GH has evicted all our CI caches of our default branch except one:
```
$ gh actions-cache list --branch master
Showing 1 of 1 cache entries in haskell/cabal

Linux-8.8.4-1c1230ca228cc03a9ee68166243af358ab3992fc 237.45 MB refs/heads/master 2 hours ago

$ date
Wed Mar 27 18:12:40 CET 2024
```

This effectively means our caching isn't working in a good way. **Because if `master` is bare of caches, every PR has to start building all the dependencies from scratch.**

Caching has to be engineered so that the latest `master` caches remain given the expected amount of PRs that are active in parallel (and generate their own caches).

In PRs #9845 and #9849 I have tried to do sensible cache engineering so that a new cache is only written if dependencies actually got updated.
However, our main workhorse `validate.yml` simply creates a new cache on **every** run (for each OS and GHC it runs) by including the `github.sha` in the cache key.
https://github.com/haskell/cabal/blob/7e085faf42a0cdb3ba19e3756590bc1f952de05b/.github/workflows/validate.yml#L111
With each cache around 300MB, you can do simple arithmetics now to see that our 10GB limit is busted all the time (even 30 GB wouldn't make much of a difference).

E.g. these are the 46 caches by **one** currently active PR:

$ gh actions-cache list -L 100 --branch refs/pull/9718/merge

Showing 46 of 46 cache entries in haskell/cabal


Linux-8.8.4-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                      257.98 MB  refs/pull/9718/merge  6 hours ago

macOS-8.6.5-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                      212.56 MB  refs/pull/9718/merge  6 hours ago

macOS-9.2.8-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                      204.08 MB  refs/pull/9718/merge  6 hours ago

macOS-8.10.7-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                     209.47 MB  refs/pull/9718/merge  6 hours ago

macOS-9.0.2-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                      206.74 MB  refs/pull/9718/merge  6 hours ago

macOS-8.8.4-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                      212.92 MB  refs/pull/9718/merge  6 hours ago

macOS-9.4.8-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                      186.53 MB  refs/pull/9718/merge  6 hours ago

macOS-9.6.3-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                      174.85 MB  refs/pull/9718/merge  6 hours ago

macOS-9.8.1-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                      175.46 MB  refs/pull/9718/merge  6 hours ago

Windows-9.2.8-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                    276.43 MB  refs/pull/9718/merge  6 hours ago

Windows-9.0.2-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                    270.71 MB  refs/pull/9718/merge  6 hours ago

Windows-9.4.8-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                    241.21 MB  refs/pull/9718/merge  6 hours ago

Windows-9.6.3-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                    224.59 MB  refs/pull/9718/merge  6 hours ago

Windows-9.8.1-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                    223.40 MB  refs/pull/9718/merge  7 hours ago

Linux-9.2.8-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                      229.11 MB  refs/pull/9718/merge  7 hours ago

Linux-8.10.7-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                     227.63 MB  refs/pull/9718/merge  7 hours ago

Linux-8.8.4-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                      227.29 MB  refs/pull/9718/merge  7 hours ago

Linux-9.0.2-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                      227.26 MB  refs/pull/9718/merge  7 hours ago

Linux-8.6.5-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                      279.99 MB  refs/pull/9718/merge  7 hours ago

Linux-9.4.8-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                      216.40 MB  refs/pull/9718/merge  7 hours ago

Linux-9.8.1-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                      177.89 MB  refs/pull/9718/merge  7 hours ago

Linux-9.6.3-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                      177.28 MB  refs/pull/9718/merge  7 hours ago

bootstrap-Linux-8.10.7-20221115-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3  279.37 MB  refs/pull/9718/merge  7 hours ago

bootstrap-Linux-9.0.2-20221115-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3   270.39 MB  refs/pull/9718/merge  7 hours ago

Linux-9.2.8-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3                      326.73 MB  refs/pull/9718/merge  7 hours ago

bootstrap-Linux-9.4.8-20221115-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3   267.49 MB  refs/pull/9718/merge  7 hours ago

bootstrap-Linux-9.2.8-20221115-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3   248.42 MB  refs/pull/9718/merge  7 hours ago

bootstrap-Linux-9.8.1-20221115-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3   201.04 MB  refs/pull/9718/merge  7 hours ago

bootstrap-Linux-9.6.4-20221115-aa5d3e49b1736ef89bfbc4f26c7875ad653264d3   202.54 MB  refs/pull/9718/merge  7 hours ago

bootstrap-Linux-9.8.1-20221115-5d6f3f3a6464c1ff23b625ac924fbb125d580a97   201.04 MB  refs/pull/9718/merge  7 hours ago

bootstrap-Linux-9.2.8-20221115-5d6f3f3a6464c1ff23b625ac924fbb125d580a97   248.41 MB  refs/pull/9718/merge  7 hours ago

bootstrap-Linux-9.6.4-20221115-5d6f3f3a6464c1ff23b625ac924fbb125d580a97   202.55 MB  refs/pull/9718/merge  7 hours ago

Linux-fix-whitespace-0.1                                                  3.01 MB    refs/pull/9718/merge  7 hours ago

bootstrap-Linux-9.4.8-20221115-5d6f3f3a6464c1ff23b625ac924fbb125d580a97   267.49 MB  refs/pull/9718/merge  7 hours ago

bootstrap-Linux-9.0.2-20221115-5d6f3f3a6464c1ff23b625ac924fbb125d580a97   270.39 MB  refs/pull/9718/merge  7 hours ago

bootstrap-Linux-8.10.7-20221115-5d6f3f3a6464c1ff23b625ac924fbb125d580a97  279.36 MB  refs/pull/9718/merge  7 hours ago

bootstrap-Linux-9.8.1-20221115-016216c7e7e7f383ddd2c5497fa737640f0b792b   201.06 MB  refs/pull/9718/merge  8 hours ago

bootstrap-Linux-9.6.4-20221115-016216c7e7e7f383ddd2c5497fa737640f0b792b   202.54 MB  refs/pull/9718/merge  8 hours ago

bootstrap-Linux-8.10.7-20221115-016216c7e7e7f383ddd2c5497fa737640f0b792b  279.37 MB  refs/pull/9718/merge  8 hours ago

bootstrap-Linux-9.2.8-20221115-016216c7e7e7f383ddd2c5497fa737640f0b792b   248.41 MB  refs/pull/9718/merge  8 hours ago

bootstrap-Linux-9.4.8-20221115-016216c7e7e7f383ddd2c5497fa737640f0b792b   267.48 MB  refs/pull/9718/merge  8 hours ago

bootstrap-Linux-9.0.2-20221115-016216c7e7e7f383ddd2c5497fa737640f0b792b   270.39 MB  refs/pull/9718/merge  8 hours ago

Linux-9.2.8-016216c7e7e7f383ddd2c5497fa737640f0b792b                      326.72 MB  refs/pull/9718/merge  a day ago

bootstrap-Linux-8.10.7-20221115-b171551765807a9594f9724ffec82dd49457491e  279.36 MB  refs/pull/9718/merge  a day ago

bootstrap-Linux-9.6.4-20221115-b171551765807a9594f9724ffec82dd49457491e   202.55 MB  refs/pull/9718/merge  a day ago

bootstrap-Linux-9.8.1-20221115-b171551765807a9594f9724ffec82dd49457491e   201.04 MB  refs/pull/9718/merge  a day ago

These alone bust the 10GB limit.

Including `github.sha` in the cache key is a nice solution to cache aging for small projects that have effectively infinite cache space (e.g. if your caches are 10MB and you have 10GB available), but for sizeable project like `cabal` this isn't a solution.
For `Agda` I spent days in cache engineering so that the goals are met (staying in the limit even with a couple of parallel PRs).

The current caching philosophy seems to originate from https://github.com/haskell/cabal/pull/7952/files#r807953232
- #7952 @andreabedini