update syntax

Author: parthea

.github/workflows/tests.yaml

@@ -75,7 +75,7 @@ jobs:
       # Run showcase tests on the lowest and highest supported runtimes
       matrix:
         # TODO(https://github.com/googleapis/gapic-generator-python/issues/2121) Remove `showcase_w_rest_async` target when async rest is GA.
-        python: [${{ vars.OLDEST_PYTHON }}, ${{ vars.NEWEST_STABLE_PYTHON }}]
+        python: ["${{ vars.OLDEST_PYTHON }}", "${{ vars.NEWEST_STABLE_PYTHON }}"]
         target: [showcase, showcase_w_rest_async]
         logging_scope: ["", "google"]