Improve CI build and test times

**Without cache hit** [source](https://github.com/astral-sh/ruff/actions/runs/6670980068)

Compile time for Linux tests 7m 23s
Compile time for Windows tests 8m 56s
Runtime for docs check 2m 33s

**With cache hit** [source](https://github.com/astral-sh/ruff/actions/runs/6671183579)

Compile time for Linux tests 3m 13s
Compile time for Windows tests 4m 48s
Runtime for docs check 35s

CI takes an additional 2 minutes to run the tests in both environments

We only check documentation on Linux.

It'd be great if these were faster. I'm not sure it's possible but I'd love to hear ideas.

Start by reviewing the linked GitHub Actions runs and comparing the cache-hit and cache-miss timings for Linux, Windows, documentation checks, and tests. Identify which CI steps account for the reported delays; done means a documented, measured reduction in those build or test times without losing coverage.

Written by the indexing model from the issue text.