fix(demo): Fix default demo index versions (#4116)

The script for deploying the demo version index page has a function
that chooses which versions to display. This script has a special
mode meant for testing that shows a default list of demo versions.
This mode was only supposed to trigger when run locally, but the
check did not work properly, so the demo version index was deploying
with a very out-of-date list of versions.
This fixes that check.

Closes #4074

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app-engine/demo-version-index/main.py

@@ -112,7 +112,7 @@ def version_key(version):
   return version_tuple + [suffix]
 
 def get_appengine_versions():
-  if os.getenv('SERVER_SOFTWARE', '').startswith('Google App Engine/'):
+  if os.getenv('GAE_ENV', '').startswith('standard'):
     # NOTE: this doesn't return anything useful in a local dev server.
     return google.appengine.api.modules.modules.get_versions()