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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](https://github.com/rust-lang/rust/issues/131744).
