Re-implement Nix CI workflow on GitHub instead of CircleCI #130

Job	Run time
packaging (macos-12, 3.9)	1s
integration (ubuntu-20.04, 3.11, false)	3m 40s
packaging (windows-latest, 3.9)	1m 53s
coverage (macos-12, 3.12)	1s
integration (macos-12, 3.11, false)	1s
coverage (ubuntu-latest, pypy-3.9)	3m 42s
integration (windows-latest, 3.11, false)	3m 27s
packaging (ubuntu-latest, 3.9)	1m 19s
coverage (ubuntu-latest, 3.12)	3m 41s
integration (ubuntu-20.04, 3.12, true)	2m 58s
coverage (windows-latest, 3.12)	3m 26s
finish-coverage-report	1s
	24m 10s

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