Fix Pixi caching in CI (#321) #981

Job	Run time
cache-pixi-lock	2m 50s
Unit tests: ubuntu-latest \| pixi run -e test-latest tests	2m 26s
Unit tests: macos-14 \| pixi run -e test-latest tests	2m 32s
Unit tests: ubuntu-latest \| pixi run -e test-py310 tests	2m 14s
Unit tests: windows-latest \| pixi run -e test-latest tests	7m 33s
	17m 35s

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