Use github mirrors by default

The github action runs on github infra, we should use github mirrors.

This setting can be changed back to opendev.org by setting an override
like this:

    conf_overrides: |
      GIT_BASE=https://opendev.org

Author: mandre

action.yaml

@@ -57,6 +57,7 @@ runs:
         LOGFILE=${{ inputs.log_dir }}/devstack.log
         USE_PYTHON3=True
         INSTALL_TEMPEST=False
+        GIT_BASE=https://github.com
         EOF
         # horizon does not work in github action, permission error
         # dstat log takes too much memory to be stored by log artifacts