Fix objc tests pull request failures

tools/internal_ci/helper_scripts/prepare_build_macos_rc

@@ -25,7 +25,7 @@ export GOOGLE_APPLICATION_CREDENTIALS=${KOKORO_GFILE_DIR}/GrpcTesting-d0eeee2db3
 
 # If this is a PR using RUN_TESTS_FLAGS var, then add flags to filter tests
 if [ -n "$KOKORO_GITHUB_PULL_REQUEST_NUMBER" ]; then
-  export RUN_TESTS_FLAGS="$RUN_TESTS_FLAGS --filter_pr_tests --base_branch origin/$KOKORO_GITHUB_PULL_REQUEST_TARGET_BRANCH"
+  export RUN_TESTS_FLAGS="--filter_pr_tests --base_branch origin/$KOKORO_GITHUB_PULL_REQUEST_TARGET_BRANCH $RUN_TESTS_FLAGS"
 fi
 
 set +ex  # rvm script is very verbose and exits with errorcode