Add environment variable query
Generally, `rustc` prefers command-line arguments, but in some cases, an environment variable really is the most sensible option. We should make sure that this works properly with the compiler's change-tracking mechanisms, such that changing the relevant environment variable causes a rebuild.
This PR is a first step forwards in doing that.
Part of the work needed to do https://github.com/rust-lang/rust/issues/118204, see https://github.com/rust-lang/rust/pull/129342 for some discussion.
r? ``@petrochenkov``
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