Ignore empty RUSTC_WRAPPER in bootstrap

[Skepfyr]

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.

[rustbot]

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[clubby789]

I can only shadow those locally with an empty string, I can't unset it entirely

Does env -u RUSTC_WRAPPER ... not work here?

[Skepfyr]

So, it does, but it's a massive pain to use. I'm currently trying to use direnv to unset it for rust-analyzer and any random commands I running, remembering to prefix everything with env -u would be somewhat annoying.

[clubby789]

I guess this makes sense, at least for consistency with Cargo
@bors r+ rollup

[bors]

📌 Commit c7003f5 has been approved by clubby789

It is now in the queue for this repository.