action: use a venv to prevent PEP 668 errors #306

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Job	Run time
selftest-runner-python (ubuntu-latest)	23s
selftest (ubuntu-latest)	19s
selftest-runner-python (ubuntu-24.04)	19s
selftest-release-signing-artifacts-no-op (ubuntu-latest)	19s
selftest (macos-latest)	25s
selftest-whitespace (ubuntu-latest)	21s
selftest-release-signing-artifacts-no-op (macos-latest)	16s
selftest (windows-latest)	1m 0s
selftest-whitespace (macos-latest)	20s
selftest-release-signing-artifacts-no-op (windows-latest)	46s
selftest (ubuntu-24.04)	29s
selftest-whitespace (windows-latest)	48s
selftest-staging	19s
selftest-glob	27s
selftest-xfail-glob-input-expansion	17s
selftest-xfail-invalid-inputs (--this-should-not-work)	18s
selftest-xfail-invalid-inputs (/tmp/extremely-nonexistent-file)	22s
selftest-custom-paths	19s
selftest-glob-multiple	24s
selftest-identity-token	19s
selftest-upload-artifacts	20s
selftest-verify	19s
selftest-xfail-verify-missing-options (true)	16s
selftest-xfail-verify-missing-options (true, https://token.actions.githubusercontent.com)	19s
selftest-xfail-verify-missing-options (true, https://github.com/sigstore/gh-action-sigstore-pytho...	20s
selftest-xfail-verify-missing-options (false, https://token.actions.githubusercontent.com)	18s
selftest-xfail-verify-missing-options (false, https://github.com/sigstore/gh-action-sigstore-pyth...	19s
all-selftests-pass	4s
	10m 45s

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