This also includes some little syntactical fixes.
The commit is needed since colmena works mostly with nix' flake
experiemental feature. It attempts to find a locked version of nixpkgs,
which wasn't there yet. By using the standard name "flake.nix", nix
automatically picks up the file and generates a `flake.lock` if it
doesn't exist yet. This brings us one step closer to a buildable system.
