rustbuild: skip filecheck check if codegen tests are disabled

[japaric]

to match the behavior of the old Makefile-based build system

closes #35752

r? @alexcrichton

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[alexcrichton]

I think this may also want to disable the codegen tests as well? It's ok to just return early from that "step" in this case.

[japaric]

It's ok to just return early from that "step" in this case.

@alexcrichton Does the last commit look OK?

[alexcrichton]

@bors: r+ b8ebc13

[bors]

⌛ Testing commit b8ebc13 with merge b17fa8b...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-no-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-no-mir, auto-win-msvc-64-opt-rustbuild
Approved by: alexcrichton
Pushing b17fa8b to master...