.github/workflows: skip net/dnsfallback go generate check on release branch

We don't want to force ourselves to update the DERP list
every time we want to cut a new release.
Having an outdated DERP list on release branches is OK.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

.github/workflows/go_generate.yml

@@ -28,7 +28,14 @@ jobs:
         run: |
           mkdir gentools
           go build -o gentools/stringer golang.org/x/tools/cmd/stringer
-          PATH="$PATH:$(pwd)/gentools" go generate ./...
+          if [[ "${{github.ref}}" == release-branch/* ]]
+          then
+            pkgs=$(go list ./... | grep -v dnsfallback)
+          else
+            pkgs=$(go list ./...)
+          fi
+          echo pkgs: $pkgs
+          PATH="$PATH:$(pwd)/gentools" go generate $pkgs
           echo
           echo
           git diff --name-only --exit-code || (echo "The files above need updating. Please run 'go generate'."; exit 1)