Move codspeed tox installation to earlier step

[seddonym]

Speeds up the benchmarking step job in CI.

Prior to this, we would run pytest via tox -ecodspeed. This meant that codspeed would run the installation of packages under valgrind, which is much slower.

This is a quick-and-dirty approach which changes the codspeed action to use uv to install the packages. This seems to reduce the time from around 17 minutes to 10 minutes. For some reason it's still installing reinstalling grimp, but it's better than it was.

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #205 will improve performances by 43.73%

_{Comparing move-codspeed-tox-installation-to-earlier-step (52dc1f8) with master (a318714)}

🎉 Hooray! pytest-codspeed just leveled up to 3.2.0!

A heads-up, this is a breaking change and it might affect your current performance baseline a bit. But here's the exciting part - it's packed with new, cool features and promises improved result stability 🥳!
Curious about what's new? Visit our releases page to delve into all the awesome details about this new version.

Summary

⚡ 3 improvements
✅ 18 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	test_copy_graph	80.6 ms	56.1 ms	+43.73%
⚡	test_find_descendants	41.5 ms	36.2 ms	+14.7%
⚡	test_find_upstream_modules	3.9 ms	3.4 ms	+14.92%