ci/nixpkgs-vet: ignore .github/ and ci/ folders

nixpkgs-vet doesn't care about our CI infrastructure, so ignoring these
files will lead to more cache hits when iterating on CI related PRs.

ci/nixpkgs-vet.nix

@@ -13,7 +13,10 @@ let
     with lib.fileset;
     path:
     toSource {
-      fileset = (gitTracked path);
+      fileset = difference (gitTracked path) (unions [
+        (path + /.github)
+        (path + /ci)
+      ]);
       root = path;
     };