Improve Nix in CI #288

Job	Run time
nix-based (with-nixpkgs)	2m 7s
opam-based (ubuntu, 4.11.x)	10m 37s
opam-based (ubuntu, 4.11.x, lower-bounds, no-doc)	8m 23s
apt-based	44s
opam-based (ubuntu, 4.12.x)	7m 56s
nix-based (with-opam-nix)	9m 24s
docker-based (alpine, deploy-image)	7m 23s
opam-based (ubuntu, 4.13.x)	8m 18s
docker-based (archlinux)	5m 49s
opam-based (ubuntu, 4.14.x, lower-bounds, no-doc)	9m 17s
docker-based (centos, no-tests)	5m 10s
opam-based (ubuntu, 4.14.x, deploy-doc)	8m 1s
docker-based (debian)	5m 34s
nix-flake-checks	2h 36m 0s
opam-based (ubuntu, 4.14.1, bytecode)	10m 36s
docker-based (debian-testing)	6m 40s
opam-based (macos, 4.14.x)	13m 24s
docker-based (debian-unstable)	5m 28s
opam-based (windows, 4.14.x, no-tests)	11m 50s
docker-based (fedora)	6m 0s
opam-based (ubuntu, 5.0.x)	8m 41s
docker-based (opensuse)	6m 20s
docker-based (oraclelinux)	5m 55s
docker-based (ubuntu)	5m 36s
docker-based (ubuntu-lts)	5m 24s
	5h 30m 37s

Provide feedback