CI: for fast CI dev flow - build only changed workflows

**Description:**

Contributions to the CI now have to pass a `{test, bench, nix}` workflows, which takes ~2 hours per loop & keeps those runners occupied & devs waiting vacuously.

**Describe the solution you'd like**

(probably by configuring `skip-duplicate-actions`): Changes to according CI workflow should trigger mostly only that workflow & maybe related ones.

**Additional context**

I tried to enumerate files directly in https://github.com/haskell/haskell-language-server/pull/2506/commits/d7c1a5a2bb58381959a6373f8589ca7ce9cd2481, which in https://github.com/haskell/haskell-language-server/pull/2506#issuecomment-997397938 is described & has a link to the build.

Later I changed `caching` only by changing docs & CI still ran into https://github.com/haskell/haskell-language-server/pull/2506#issuecomment-997424955.

Start by reading the CI workflow configuration and the skip-duplicate-actions setup discussed in pull request #2506, including its linked build and comments. Reproduce the caching-only change described there, then verify that changes trigger only the affected workflow and any necessary related workflows instead of the full test, bench, and nix set.

Written by the indexing model from the issue text.