fix: Don't write to tmpdir in CI #276

Job	Run time
lockfile	15s
Test (ubuntu-latest, stable)	20s
Docs	19s
rustfmt	20s
Coverage	29s
Check MSRV (ubuntu-latest)	24s
clippy	26s
Test (macos-latest, stable)	33s
Minimal versions (ubuntu-latest)	33s
Test (windows-latest, stable)	2m 10s
CI	4s
	5m 53s

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