Bump actions/checkout from 3 to 4 #278

Job	Run time
nix-based (with-nixpkgs)	2m 33s
nix-based (with-opam-nix)	2m 37s
apt-based	2m 41s
docker-based (alpine, deploy-image)	2m 36s
docker-based (archlinux)	2m 38s
docker-based (centos, no-tests)	2m 58s
docker-based (debian)	2m 38s
docker-based (debian-testing)	2m 48s
docker-based (debian-unstable)	2m 44s
docker-based (fedora)	2m 36s
docker-based (opensuse)	2m 56s
docker-based (oraclelinux)	2m 41s
docker-based (ubuntu)	2m 41s
docker-based (ubuntu-lts)	2m 50s
nix-flake-checks	2m 41s
opam-based (ubuntu, 4.11.x)	2m 34s
opam-based (ubuntu, 4.11.x, lower-bounds, no-doc)	2m 54s
opam-based (ubuntu, 4.12.x)	2m 32s
opam-based (ubuntu, 4.13.x)	2m 41s
opam-based (ubuntu, 4.14.x, lower-bounds, no-doc)	2m 47s
opam-based (ubuntu, 4.14.x, deploy-doc)	2m 42s
opam-based (ubuntu, 4.14.1, bytecode)	2m 37s
opam-based (macos, 4.14.x)	2m 53s
opam-based (windows, 4.14.x, no-tests)	2m 50s
opam-based (ubuntu, 5.0.x)	2m 42s
	1h 7m 50s

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