chore(python): use cov_level in unittest gh action (#1335)

Author: parthea

synthtool/gcp/templates/python_library/.github/workflows/unittest.yml

@@ -54,4 +54,4 @@ jobs:
     - name: Report coverage results
       run: |
         coverage combine .coverage-results/.coverage*
-        coverage report --show-missing --fail-under=100
+        coverage report --show-missing --fail-under={{ cov_level if cov_level != None else 100 }}