Bump `actions/checkout` to `v6` #842

Job	Run time
Test (ubuntu-latest, net9.0, x64, Release)	1m 45s
Test (windows-latest, net8.0, x64, Release)	3m 33s
Test (windows-latest, net472, x64, Release)	2m 23s
Test (windows-latest, net48, x64, Release)	3m 13s
Test (ubuntu-latest, net8.0, x64, Release)	1m 52s
Test (windows-latest, net9.0, x64, Release)	3m 38s
	16m 24s

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