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 improve performance by 13.8%

⚡ 1 improved benchmark
✅ 104 untouched benchmarks
⏩ 1 skipped benchmark¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	bench_pyclass_create	4.6 µs	4 µs	+13.8%

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_{Comparing davidhewitt:build-script-coverage (23e26e5) with main (8a00673)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[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.