Move default output root on macOS to $HOME/Library/Caches

[aaronsky]

Description of the feature request:

Forking from #25167 and its implementation in #25205, it was requested during code review that the default output root on macOS be moved from the historically relevant /private/var/tmp to the more conventional $HOME/Library/Caches. This issue serves to track that implementation.

As a relatively new contributor, it's unclear if this is would be an incompatible change, though based on the fact I can't see any incompatible startup options, I am assuming it's either atypical or not supported in the release model. Happy to roll this out in any way a contributor sees fit for it to be.

Which category does this issue belong to?

Core

What underlying problem are you trying to solve with this feature?

No response

Which operating system are you running Bazel on?

macOS 15.3.1

What is the output of bazel info release?

release 8.0.1

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse HEAD ?

[email protected]:aaronsky/bazel.git
95bbed03758a7651916d0708a892f5342a0421c5

Have you found anything relevant by searching the web?

No response

Any other information, logs, or outputs that you want to share?

No response