Bump actions/checkout from 3 to 4 #285

Job	Run time
nix-flake-checks	40m 55s
apt-based	57s
nix-based (with-nixpkgs)	1m 45s
docker-based (alpine, deploy-image)	6m 43s
nix-based (with-opam-nix)	8m 18s
docker-based (archlinux)	5m 57s
opam-based (ubuntu, 4.11.x)	9m 28s
docker-based (centos, no-tests)	4m 42s
opam-based (ubuntu, 4.11.x, lower-bounds, no-doc)	8m 23s
docker-based (debian)	7m 21s
opam-based (ubuntu, 4.12.x)	11m 3s
docker-based (debian-testing)	5m 37s
opam-based (ubuntu, 4.13.x)	8m 19s
docker-based (debian-unstable)	6m 54s
opam-based (ubuntu, 4.14.x, lower-bounds, no-doc)	10m 15s
docker-based (fedora)	5m 36s
opam-based (ubuntu, 4.14.x, deploy-doc)	8m 11s
docker-based (opensuse)	7m 17s
opam-based (ubuntu, 4.14.1, bytecode)	10m 21s
docker-based (oraclelinux)	7m 34s
opam-based (macos, 4.14.x)	13m 47s
docker-based (ubuntu)	5m 39s
docker-based (ubuntu-lts)	5m 30s
opam-based (windows, 4.14.x, no-tests)	11m 23s
opam-based (ubuntu, 5.0.x)	9m 56s
	3h 41m 51s

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