Rollup merge of #132267 - onur-ozkan:rustc-if-unchanged-force-library, r=Kobzol

force-recompile library changes on download-rustc="if-unchanged"

This makes the download-rustc="if-unchanged" option more functional and useful for library developers.

Implements the second item from [this tracking issue](rust-lang/rust#131744).

Author: workingjubilee