Move `build-dir` out of the workspace to `cache-cache-home` by default

[epage]

As a follow up to #14125, we've talked about changing the default for build-dir after a sufficient time of it being stabilized so people have a chance to test their builds and tools with it to prepare.

Open questions:

• How long do we need to wait before making the switch over?
  • See #t-cargo > build-dir tool compatibility @ 💬 for a discussion for who all may be affected
• What should the new default be?
  • fix(script): Tweak cargo script build-dir / target-dir #16086 moved the build-dir for cargo scripts to "{cargo-cache-home}/build/{workspace-path-hash}" as a proposal for what we should do for this.
  • Note that at https://www.reddit.com/r/rust/comments/1o8l3qq/cargoscript_call_for_testing/njyhipl/, someone asked for a mnemonic to be used to easily identify these build-dirs, like us adding a variable for the name of the directory the workspace is in. Granted, this would also balloon the size of the path