nixpkgs-update: add git as dependency for update scripts

eventually, it should be possible to pass git through to the
updatescripts directly within the nixpkgs-update binary, but this is
quicker for now.

build01/nixpkgs-update.nix

@@ -10,6 +10,7 @@ let
 
   nixpkgsUpdateSystemDependencies = with pkgs; [
     nix # for nix-shell used by python packges to update fetchers
+    git # used by update-scripts
     gnugrep
     curl
   ];