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

---

_{Comparing davidhewitt:build-script-coverage (818ee2c) with main (65d6a36)}

[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.