Rollup merge of #124903 - Skepfyr:rustc-wrapper, r=clubby789

Ignore empty RUSTC_WRAPPER in bootstrap

This change ignores the RUSTC_WRAPPER_REAL environment variable if it's set to the empty string. This matches cargo behaviour and allows users to easily shadow a globally set RUSTC_WRAPPER (which they might have set for non-rustc projects).

I hit this because I have RUSTC_WRAPPER set to `sccache` in my fish universal env vars, and I can only shadow those locally with an empty string, I can't unset it entirely.