CI: Only cache objects that are part of the current build

[RedworkDE]

This is the first idea from #79919 (comment)
This prevents caches from growing any larger than absolutely necessary to get effectively incremental builds in the CI, but makes selection of which cache to fetch significantly more important as the cache no longer contains any objects from previous (or potentially unrelated) builds.

Currently disabled on windows as the find command is an entirely different tool there and those builds don't have space issues rn, so I didn't go looking for how to best do this on windows for this PR. (But if desired the best way would probably to rewrite this is JS, which would also eliminated the dependency for running stuff in the post step)

[YuriSizov]

I think this is rather hacky. The idea is solid, we should try to reduce the size of the cache by cutting the excess. But this doesn't seem robust to me. Maybe we shouldn't cache .scons-cache and should do something else instead.