fix: Improve test error message for missing credentials (#1024)

* Improve test error message for missing credentials

See relevant issue #1023

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

---------

Co-authored-by: Andrew Gorcester <gorcester@google.com>
Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

noxfile.py

@@ -119,7 +119,9 @@ def system(session):
         session.skip("RUN_SYSTEM_TESTS is set to false, skipping")
     # Environment check: Only run tests if the environment variable is set.
     if not os.environ.get("GOOGLE_APPLICATION_CREDENTIALS", ""):
-        session.skip("Credentials must be set via environment variable")
+        session.skip(
+            "Credentials must be set via environment variable GOOGLE_APPLICATION_CREDENTIALS"
+        )
     # mTLS tests requires pyopenssl.
     if os.environ.get("GOOGLE_API_USE_CLIENT_CERTIFICATE", "") == "true":
         session.install("pyopenssl")