ci: include coverage from running build scripts

[davidhewitt]

This attempts to improve coverage measurement of pyo3-build-config by measuring the coverage of invocation of build.rs files.

I refactored a few common pieces about coverage measurement into helper actions so that I could re-use them from test-version-limits job.

[codspeed-hq]

Merging this PR will not alter performance

✅ 125 untouched benchmarks

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_{Comparing davidhewitt:build-script-coverage (1bb7984) with main (072d544)}

[ngoldbaum]

Thanks for working on this! I noticed this looking at our clverage output for my build configuration PR. Happy to help out if you want a hand or an eye for review.

[davidhewitt]

This depends on taiki-e/cargo-llvm-cov#493 merging & releasing first.

[davidhewitt]

Just needs https://github.com/taiki-e/install-action to have a new release to pick this up (currently 2.77.7).

[davidhewitt]

Looks like this worked. Might be useful to merge before #5807

[ngoldbaum]

I did a little AI-assisted code review and spotted a few issues. See below.

Overall looks good and I'm happy with this going in although I'm not sure about the manylinux question below.

[ngoldbaum]

Is there any way to install the needed packages on the manylinux containers? Otherwise we lose that we're checking compatibility with the manylinux ABI.

[davidhewitt]

I think I've managed to revert the manylinux stuff by forwarding appropriate env vars & cargo-llvm-cov binary into the manylinux container, will send to merge.