Bump actions/checkout from 5 to 6 #15

Job	Run time
test (ubuntu-latest, 26.3, false)	37s
test (ubuntu-latest, 27.2, false)	36s
test (windows-latest, 26.3, false)	1m 17s
test (macos-latest, 28.2, false)	2m 39s
test (ubuntu-latest, 28.2, false)	37s
test (windows-latest, 29.4, false)	1m 24s
test (ubuntu-latest, 30.1, false)	36s
test (ubuntu-latest, 29.4, false)	31s
test (windows-latest, 27.2, false)	1m 7s
test (macos-latest, 30.1, false)	2m 48s
test (windows-latest, snapshot, true)	58s
test (macos-latest, 29.4, false)	2m 34s
test (windows-latest, 30.1, false)	1m 0s
test (macos-latest, snapshot, true)	2m 33s
test (windows-latest, 28.2, false)	57s
test (ubuntu-latest, snapshot, true)	32s
	20m 46s

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