fix: await nested test cleanup and discard package artifacts

[tisonkun]

Running cargo package before saving the cache leaves unpacked sources under target/package/<crate>/tests. Cleanup currently treats these source directories as nested test build directories and attempts to open their nonexistent target and trybuild children. The two cleanup calls are not awaited, so their rejections escape the surrounding try/catch and produce unhandled ENOENT annotations. Real nested test cleanup can also outlive its caller.

Await both nested cleanup calls and discard target/package at each target-root entry point before recursively pruning build artifacts. Package archives and verification sources are excluded together, while directories named package below target roots retain the normal pruning behavior. This applies to save cleanup and restore-time timestamp pruning.

Validation:

• npm run prepare on Node.js 24.20.0: succeeds and leaves the checked-in dist/ unchanged.
• Created and verified a real dependency-free crate with cargo package --offline --allow-dirty --no-metadata, including tests/fixtures, and successfully cleaned the resulting target directory.
• git diff --check: passes.

The checked-in dist/ files are regenerated with npm run prepare. The large cache-chunk diff consists of line-ending normalization and the generated cleanup import update; ignoring whitespace isolates the code changes.