trigger rebuilds when virtualenv changes

[davidhewitt]

I realised that frequently I am attempting to test specific Python versions with commands like

UV_PYTHON=3.12 uv run cargo test --test test_compile_error

Unfortunately at the moment, even though uv recreates the venv, we don't reconfigure the build. (Full details of why are basically the same as #4882.)

I realised today that when the venv is recreated there will be a new pyvenv.cfg file created, and that will have a new mtime, so we should be able to reliably trigger a rebuild based on that.

[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:rebuild-venv-cfg (9b42f6a) with main (9c734a1)}

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]

I realised today that when the venv is recreated there will be a new pyvenv.cfg file created, and that will have a new mtime, so we should be able to reliably trigger a rebuild based on that.

Nice! This probably doesn't help my pyenv workflow which doesn't rely on virtualenvs but maybe there's similar metadata we can check for in a pyenv-managed install.

Glad to see there's a solution that will work for people who aren't doing atrocities with global Python environments like I am.