Call for Testing: Build Dir Layout v2

The Rust/Cargo team is calling for community testing of a new nightly-only build directory layout (`-Zbuild-dir-new-layout`). The change reorganizes intermediate build artifacts from a content-type-based structure to a package-name-and-hash-scoped structure. This is a stepping stone toward cross-workspace caching, automatic cleanup of stale build units, more granular locking, and improved build performance. Several libraries (assert_cmd, snapbox, trycmd, executable-path) are already fixed, while others (cli_test_dir, compiletest_rs, term-transcript, test_bin) have open issues. Users are asked to run their tests and release processes with the flag and report any issues to the tracking issue.