.github: show diff when gofmt check fails

Rather than just failing the check, also show the diff.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

.github/workflows/tests.yml

@@ -25,7 +25,7 @@ jobs:
       uses: actions/checkout@v2
     - name: Format
       if: matrix.go-version >= '1.16'
-      run: test -z $(gofmt -l .)
+      run: diff -u <(echo -n) <(gofmt -d .)
     - name: Build
       run: go build
     - name: Vet