Rollup merge of #145557 - Kobzol:rustc-link-fix, r=jieyouxu

Fix uplifting in `Assemble` step

In rust-lang/rust#145310, I removed [this line](https://github.com/rust-lang/rust/pull/145310/files#diff-5a1e05f2688d271039171a547d407d0c8a96715ee64d35562fc76b4c9a874303L2109), which adjusted the stage of the build compiler if an uplift has happened. This broke stage3+ uplifted rustc builds (rust-lang/rust#145534). I could swear I tested this in the PR, but somehow I missed it.

Instead of keeping the original returned stage, I made it more explicit by returning the actually used `build_compiler` from the `Rustc` step, and then use that in the `Assemble` step.

The changes to `RustcLink` were needed to fix `ui-fulldeps`, which apparently build a stage3 rustc, because I haven't fixed the test steps yet 😅

Hopefully we might be able to remove `RustcLink` if the approach from rust-lang/rust#144252 will work.

Should fix rust-lang/rust#145534.

r? ``@jieyouxu``

Author: Zalathar