Rollup merge of #131493 - madsmtm:avoid-redundant-linker-path, r=jiey…

…ouxu

Avoid redundant sysroot additions to `PATH` when linking

Currently, `rustc` prepends `$HOME/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/bin` to the `PATH` three times before invoking the linker, which is unnecessary, once should be enough.

Spotted this while trying to get `-Clinker-flavor=gcc` and `-Clinker-flavor=ld` closer together, not really important.

```@rustbot``` A-linkage

compiler/rustc_session/src/session.rs

@@ -454,6 +454,8 @@ impl Session {
         let bin_path = filesearch::make_target_bin_path(&self.sysroot, config::host_triple());
         let fallback_sysroot_paths = filesearch::sysroot_candidates()
             .into_iter()
+            // Ignore sysroot candidate if it was the same as the sysroot path we just used.
+            .filter(|sysroot| *sysroot != self.sysroot)
             .map(|sysroot| filesearch::make_target_bin_path(&sysroot, config::host_triple()));
         let search_paths = std::iter::once(bin_path).chain(fallback_sysroot_paths);