fix(build): Depend on GNU Parallel during build

CI builds for `master` track unstable nixpkgs releases, in which
parallel is no longer part of the standard build environment.

However, this tool is required for executing some of the tests.

default.nix

@@ -18,6 +18,7 @@ with pkgs; buildGoPackage rec {
   src = ./.;
   goPackagePath = "github.com/tazjin/kontemplate";
   goDeps = ./deps.nix;
+  buildInputs = [ parallel ];
 
   # Enable checks and configure check-phase to include vet:
   doCheck = true;