feat(github): use git for all file operations (#3806)

With Renovate’s github platform code now using git for all file system operations, we need to tell Renovate which gitAuthor to use.

If you had already configured a gitAuthor in your bot config, you do not need to make any change.

Otherwise, to keep functionality as before, you should either:
(1) configure `gitAuthor` to match the bot’s account, or
(2) recreate your bot’s personal access token to include the “user:email” permission so that the bot can retrieve the email itself

BREAKING CHANGE: GitHub bot admins should either configure gitAuthor in their config or generate a new token with “user:email” permissions.