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

[aaronsky]

Fixes #25260

Implements the request made here and moves the default output root on macOS from /private/var/tmp to $HOME/Library/Caches. May constitute an incompatible change, though on a cursory glance I didn't see any precedent for incompatible startup options in source.

[tetromino]

This is an incompatible change, but arguably one that doesn't need to be gated by a flag. We do need to make sure that

• the change is described in bazel 9 release notes
• it doesn't get cherry-picked into 8.x point releases

An additional question is what to do about the documentation at https://bazel.build/remote/output-directories - I assume many bazel 8 users will check that page (even though it's for bazel-at-head). So maybe we need phrasing along the lines of "this is how things work in bazel 9, and that is how they used to work in bazel 8 and earlier".

[tetromino]

Did some code searching.

Needs to be fixed:

• Google has a bunch of CI and admin scripts internally with /private/var/tmp hard-coded. We need to migrate them before or soon after merging this PR. (Internal tracking bug 397461058).
• Fix https://github.com/bazelbuild/continuous-integration/blob/master/macos/mac-cleanup.sh
• Fix Bazel example config at https://github.com/actions/cache/blob/main/examples.md because it looks like many people are copy-pasting it

Already fixed:

• DarwinSandboxedSpawnRunner.java already marks ~/Library/Caches as always-writable

[tetromino]

Please also update output-directories.md.

I'd suggest something like the following wording:

• The outputRoot directory defaults to ~/.cache/bazel on Linux,
  ~/Library/Caches/bazel on macOS (when using Bazel 9 and newer), and on Windows it defaults to %HOME% if
  set, else %USERPROFILE% if set, else the result of calling
  SHGetKnownFolderPath() with the FOLDERID_Profile flag set. If the
  environment variable $XDG_CACHE_HOME is set on either Linux or
  macOS, the value ${XDG_CACHE_HOME}/bazel will override the default.
  If the environment variable $TEST_TMPDIR is set, as in a test of Bazel
  itself, then that value overrides any defaults.
  • Note that Bazel 8.x and earlier on macOS used /private/var/tmp as outputRoot, and ignored $XDG_CACHE_HOME.

[aaronsky]

Thanks for the feedback @tetromino! I've addressed it in the latest commit. I'm happy to continue working with you to iterate on this as needed. I don't know if I'm the best person to work on those other identified points, since some of them are internal to Google and others seem to require some inter-repo coordination, but let me know what you want from me!