Caching code for CI is too duplicated

We have a ton of copy-pasted cache code so every adjustment or fix has to touch a lot of places and you're bound to miss one at some point.

I strongly suggest we create a composite action for that, which could be done directly in this repo, in `.github/actions`.
But if you want, it can be tackled in a separate PR :)

_Originally posted by @TimJentzsch in https://github.com/bevyengine/bevy/pull/20144#discussion_r2211433601_

Start by reviewing the duplicated cache steps in the repository's CI workflows and the discussion in PR #20144. Define the composite action under `.github/actions`, then verify that the workflows use it consistently and that CI caching behavior remains unchanged.

Written by the indexing model from the issue text.