CI: fix contribs not rebuilt if prebuilt failed

Regression from db5a4b06

extras/ci/gitlab-ci.yml

@@ -266,6 +266,8 @@ debian:
         if [ -n "$VLC_PREBUILT_CONTRIBS_URL" ]; then
             echo "Building using prebuilt contribs at $VLC_PREBUILT_CONTRIBS_URL"
             make prebuilt PREBUILT_URL="${VLC_PREBUILT_CONTRIBS_URL}" || PREBUILT_FAILED=yes
+        else
+            PREBUILT_FAILED=yes
         fi
         if [ -n "$PREBUILT_FAILED" ]; then
             make list