[ci] Use the latest version of `actions/checkout` #745

Job	Run time
build (ubuntu-latest, 4.08.x, true, true, *.opam )	7m 53s
build (ubuntu-latest, 4.08.x, false, true, *.opam )	7m 25s
build (ubuntu-latest, 4.09.x, true, true, *.opam )	8m 30s
build (ubuntu-latest, 4.09.x, false, true, *.opam )	6m 58s
build (ubuntu-latest, 4.10.x, true, true, *.opam )	7m 24s
build (ubuntu-latest, 4.10.x, false, true, *.opam )	6m 41s
build (ubuntu-latest, 4.11.x, true, true, *.opam )	7m 51s
build (ubuntu-latest, 4.11.x, false, true, *.opam )	6m 52s
build (ubuntu-latest, 4.12.x, true, true, *.opam )	8m 13s
build (ubuntu-latest, 4.12.x, false, true, *.opam )	7m 9s
build (ubuntu-latest, 4.13.x, true, true, *.opam )	5m 15s
build (ubuntu-latest, 4.13.x, false, true, *.opam )	4m 33s
build (ubuntu-latest, 4.14.x, true, true, *.opam )	4m 40s
build (ubuntu-latest, 4.14.x, false, true, *.opam )	3m 57s
build (ubuntu-latest, 5.0.x, true, true, *.opam )	4m 44s
build (ubuntu-latest, 5.0.x, false, true, *.opam )	4m 16s
build (ubuntu-latest, ocaml-variants.4.14.0+options,ocaml-option-flambda,ocaml-option-musl,ocaml-...	4m 59s
build (macos-latest, 4.14.x, true, true, *.opam )	9m 26s
build (windows-latest, 4.14.x, false, true, *.opam )	7m 10s
build (ubuntu-latest, 4.08.x, true, false, *.opam !lwt_ppx.opam )	6m 33s
build (macos-latest, 4.08.x, true, false, *.opam !lwt_ppx.opam )	7m 49s
build (windows-latest, 4.08.x, false, false, *.opam !lwt_ppx.opam )	4m 0s
	2h 22m 18s

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