Fix generating rustc docs with non-default lib directory.
If `libdir` is set in `config.toml`, then the tool to generate the rustc docs was unable to run `rustc` because it could not find the shared libraries. The solution is to set the dylib search path to include the libdir.
I changed the API of `add_rustc_lib_path` to take `Command` instead of `Cargo` to try to share the code in several places. This is how it worked before https://github.com/rust-lang/rust/pull/64316, and I think this still retains the spirit of that change.
Fixes#76702
f631293565