ci: pin all GitHub Actions to commit SHAs #799

Job	Run time
checks / readme-snippets	10s
checks / pre-commit	32s
checks / test (3.11, locked, windows-latest)	3m 7s
checks / test (3.12, locked, windows-latest)	3m 28s
checks / test (3.12, lowest-direct, windows-latest)	3m 48s
checks / test (3.12, locked, ubuntu-latest)	52s
checks / test (3.11, locked, ubuntu-latest)	49s
checks / test (3.10, lowest-direct, windows-latest)	2m 19s
checks / test (3.10, lowest-direct, ubuntu-latest)	56s
checks / test (3.12, lowest-direct, ubuntu-latest)	57s
checks / test (3.10, locked, ubuntu-latest)	49s
checks / test (3.11, lowest-direct, ubuntu-latest)	56s
checks / test (3.14, locked, ubuntu-latest)	2m 16s
checks / test (3.14, lowest-direct, windows-latest)	2m 33s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 16s
checks / test (3.13, lowest-direct, windows-latest)	2m 33s
checks / test (3.14, locked, windows-latest)	3m 21s
checks / test (3.10, locked, windows-latest)	2m 15s
checks / test (3.13, locked, windows-latest)	2m 39s
checks / test (3.11, lowest-direct, windows-latest)	3m 18s
checks / test (3.13, lowest-direct, ubuntu-latest)	51s
checks / test (3.13, locked, ubuntu-latest)	45s
all-green	4s
	40m 34s

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