CI should check that we don't build rustc_codegen_spirv more than once.

[eddyb]

See #546 for some context and ideas - especially #546 (comment).

Sadly, this is likely blocked on moving CI to use --release to build examples (which depend on "build dependency release mode rustc_codegen_spirv via the spirv-builder-using build scripts), otherwise the problem can't be reproduced in the first place (AFAIK).

[oisyn]

Superseded by #911

[eddyb]

Reopened because I think it's orthogonal to #911: we should find a way to detect redundant builds on CI regardless of what those build steps do internally.

Worst case this looks like checking that Compiling rustc_codegen_spirv appears only once in the build logs (though build script stdout/stderr is hidden by default so it would have to search in target/**/build/**/std{out,err} too).

Maybe all we need is a RUSTC_WRAPPER that adds the command to a "log file" and then we can check that e.g. --crate-name rustc_codegen_spirv appears only once (we'd have to make sure this gets propagated to build scripts tho).