Bump actions/checkout from 3 to 4 #89

Job	Run time
build (3.1 / ubuntu-latest)	16s
build (3.1 / macos-latest)	30s
build (3.1 / windows-latest)	1m 58s
build (3.0 / ubuntu-latest)	13s
build (3.0 / macos-latest)	29s
build (3.0 / windows-latest)	1m 9s
build (2.7 / ubuntu-latest)	30s
build (2.7 / macos-latest)	1m 2s
build (2.7 / windows-latest)	1m 45s
build (head / ubuntu-latest)	19s
build (head / macos-latest)	40s
build (truffleruby-head / ubuntu-latest)	5m 42s
build (truffleruby-head / macos-latest)	6m 3s
build (mswin / windows-latest)	1m 52s
build (mingw / windows-latest)	1m 42s
	24m 10s

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