Grid definitions should explicitly specify the nixpkgs version they use.

From @jcalderone (discussion)

I did two local morph builds for the testing grid. First on develop and then on this branch. Quite surprisingly, morph built the same thing for each. How could this be, with this nixpkgs update in the branch?

To make the grid definitions more reproducible, they should explicitly define which nixpkgs version they want, rather than using whatever NIX_PATH is defined in the environment. This means a developer or script doesn't need to start a fresh nix-shell to pickup changes to the pinned nixpkgs version.