chore: update `actions/checkout` to `v4` #174

Job	Run time
selftest-pipaudit-fail	7s
selftest-environment	11s
selftest-pyproject	14s
selftest-requirements (ubuntu-latest)	13s
selftest-virtualenv (true)	12s
selftest-requirements (windows-latest)	45s
selftest-virtualenv (false)	13s
all-selftests-pass	3s
	1m 58s

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