ci: add variable to select gitlab sync or stay on github master

Author: sobuch

.gitlab/ci/downstream/build-test-app-template.yml

@@ -25,7 +25,7 @@
   if [ "${TEST_APP_IDF_DOCKER_IMAGE}" = "${TEST_APP_IDF_LATEST_DOCKER_IMAGE}" ]; then
     if  [ -n "${TEST_APP_IDF_MASTER_BRANCH:-}" ]; then
       TEST_APP_CUSTOM_BRANCH="$TEST_APP_IDF_MASTER_BRANCH"
-    else
+    elif [ "${TEST_APP_IDF_LATEST_SYNC_FROM_GITLAB}" = "1" ]; then
       # Checkout from gitlab master
       TEST_APP_CUSTOM_BRANCH="master"
     fi