Rollup merge of #104076 - ozkanonur:fix-ci-rustc-sysroot, r=jyn514

fix sysroot issue which appears for ci downloaded rustc

Currently when compiler is downloaded rather than compiled, sysroot is being `ci-rustc-sysroot` because of https://github.com/rust-lang/rust/blob/7eef946fc0e0eff40e588eab77b09b287accbec3/src/bootstrap/compile.rs#L1125-L1131 this.

And rustdoc is overriding the downloaded one at the end of the process.

With the condition I add, we simply check if the current compiler stage is target build stage, if so use the proper sysroot instead of `ci-rustc-sysroot`.

Resolves #103206

diff --cc src/bootstrap/compile.rs
Simple merge