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

See #546 for some context and ideas - especially https://github.com/EmbarkStudios/rust-gpu/pull/546#issuecomment-809089160.

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).