Update Nix configuration #292

Job	Run time
apt-based	1m 2s
nix-flake-checks	1m 44s
nix-based (with-nixpkgs)	1m 55s
docker-based (alpine, deploy-image)	5m 37s
nix-based (with-opam-nix)	7m 46s
opam-based (ubuntu, 4.11.x)	9m 38s
docker-based (archlinux)	6m 13s
opam-based (ubuntu, 4.11.x, lower-bounds, no-doc)	8m 11s
docker-based (centos, no-tests)	4m 55s
opam-based (ubuntu, 4.12.x)	8m 51s
docker-based (debian)	5m 35s
opam-based (ubuntu, 4.13.x)	9m 47s
docker-based (debian-testing)	6m 37s
opam-based (ubuntu, 4.14.x, lower-bounds, no-doc)	9m 41s
docker-based (debian-unstable)	6m 15s
opam-based (ubuntu, 4.14.x, deploy-doc)	8m 22s
docker-based (fedora)	5m 46s
opam-based (ubuntu, 4.14.1, bytecode)	11m 38s
docker-based (opensuse)	7m 43s
opam-based (macos, 4.14.x)	11m 55s
docker-based (oraclelinux)	6m 10s
opam-based (windows, 4.14.x, no-tests)	12m 25s
docker-based (ubuntu)	5m 33s
opam-based (ubuntu, 5.0.x)	9m 24s
docker-based (ubuntu-lts)	5m 25s
	2h 58m 8s

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