ci: reduce test workflow overhead

[jezdez]

Description

First of three stacked PRs reducing CI test overhead.

This PR:

• moves the coverage options from global pytest configuration into the Linux, Windows, and macOS test steps
• keeps coverage enabled in CI while avoiding it for local pytest and benchmark runs
• trims the Linux disk-cleanup list to the entries that recover meaningful space

The test selection, xdist scheduling, and retry policy are unchanged. Test jobs continue to fetch full git history because hatch-vcs uses it to calculate the project version.

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests? — workflow and test configuration only
• Add / update outdated documentation? — no user-facing documentation affected

[codspeed-hq]

Merging this PR will not alter performance

✅ 5 untouched benchmarks

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_{Comparing ci/test-speed-hygiene (b4f0aa7) with main (fa6a170)¹}

Footnotes

1. No successful run was found on main (e7ad913) during the generation of this report, so fa6a170 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩