rustbuild: Switch back to using hard links

The `copy` function historically in rustbuild used hard links to speed up the
copy operations that it does. This logic was backed out, however, in #39518 due
to a bug that only showed up on Windows, described in #39504. The cause
described in #39504 happened because Cargo, on a fresh build, would overwrite
the previous artifacts with new hard links that Cargo itself manages.

This behavior in Cargo was fixed in rust-lang/cargo#4390 where it no longer
should overwrite files on fresh builds, opportunistically leaving the filesystem
intact and not touching it.

Hopefully this can help speed up local builds by doing fewer copies all over the
place!

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