github: stop quietening grep for images containing RIFF

... if it exists it would be nice to know in what!

.github/workflows/test.yml

@@ -28,7 +28,7 @@ jobs:
           ./Sphinxsetup.sh
       - name: Check image formats
         run: |
-          find ./images/ -exec file {} + | grep RIFF -vzq
+          find ./images/ -exec file {} + | grep RIFF -vz
       - name: Check python3 flake8 formatting
         run : |
           set -eux -o pipefail