nix: Don't warn when git tree is dirty

I run `git status` frequently enough that I already know when the tree
is dirty. Additionally, the starship prompt shows the git status anyway.

modules/nix.nix

@@ -7,6 +7,7 @@
     settings = {
       experimental-features = [ "nix-command" "flakes" "repl-flake" ];
       auto-optimise-store = true;
+      warn-dirty = false;
     };
   };
 }