Export stack image if not provided as input

Author: dariaguy

.github/actions/run-tests/action.yml

@@ -109,6 +109,9 @@ runs:
 
         else
           echo "Using redis CE for module tests"
+          if [[ -z "${CLIENT_LIBS_TEST_STACK_IMAGE_TAG}" ]]; then
+            export CLIENT_LIBS_TEST_STACK_IMAGE_TAG=$REDIS_VERSION
+          fi
           echo "REDIS_MOD_URL=redis://127.0.0.1:6379" >> $GITHUB_ENV
           invoke devenv --endpoints all
         fi