pkgs.omp: init at 18.11.2
nixos-unstable has omp 18.11.0, whose fetchFromGitHub hash no longer
matches the tarball GitHub serves (Hydra rejects the output as well), so
coaxmetal, neoprism and barnacle fail to build. Pin the version from
nixpkgs master (18.11.2: new src and cargoDeps hashes, node_modules hash
unchanged) as an override of nixpkgs' omp. The assert breaks evaluation
as soon as nixpkgs moves away from 18.11.0, so the pin gets dropped with
the next flake update that brings a fixed omp.