.github/workflows: make go_generate check detect new files

Updates #17957

Change-Id: I904fd5b544ac3090b58c678c4726e7ace41a52dd
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

.github/workflows/test.yml

@@ -703,6 +703,7 @@ jobs:
       run: |
         pkgs=$(./tool/go list ./... | grep -Ev 'dnsfallback|k8s-operator|xdp')
         ./tool/go generate $pkgs
+        git add -N . # ensure untracked files are noticed
         echo
         echo
         git diff --name-only --exit-code || (echo "The files above need updating. Please run 'go generate'."; exit 1)