ci: make gofmt check fail for a non-empty diff (#7131)

Fixes #7130.

Change-Id: If47eb472ea98a8d8b250c5c681c7862d252645fb
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

.github/workflows/static-analysis.yml

@@ -24,7 +24,9 @@ jobs:
       with:
         go-version-file: go.mod
     - name: Run gofmt (goimports)
-      run: go run golang.org/x/tools/cmd/goimports -d --format-only .
+      run: |
+        OUT=$(go run golang.org/x/tools/cmd/goimports -d --format-only .)
+        [ -z "$OUT" ] || (echo "Not gofmt'ed: $OUT" && exit 1)
     - uses: k0kubun/action-slack@v2.0.0
       with:
         payload: |