Revert "always enabled whether or not in GitHub Actions workflow"

This reverts commit d3c0d24.

Author: utgwkk

pytest_github_actions_annotate_failures/plugin.py

@@ -1,6 +1,11 @@
 import os
 
 def pytest_runtest_logreport(report):
+    # enable only in a workflow of GitHub Actions
+    # ref: https://help.github.com/en/actions/configuring-and-managing-workflows/using-environment-variables#default-environment-variables
+    if not os.environ.get('GITHUB_ACTIONS'):
+        return
+
     if report.outcome != 'failed':
         return