[BUG] mergify can't rebase "random" user's PRs

[MattSturgeon]

Description

Apparently it is deprecated, and we should use the update_bot_account option:

For the queue action, public repositories accepting pull requests from users outside their organization must set a update_bot_account

Looking at the docs for update_bot_account:

For rebasing branches, Mergify might have to impersonate a GitHub user. You can specify the account to use with this option.
If no update_bot_account is set, Mergify uses the author of the PR. The user account must have already been logged in Mergify dashboard once.
For update_method: merge this option will be ignored in order for Mergify to not reset the merge queue, because of the merge commit done has a person instead of a bot.

I don't know if there is a way to continue using the PR author, unless they are not an org-member?

Since it's a template type maybe there's a way we can do this with a template expression?

Example

https://github.com/nix-community/nixvim/pull/1785/checks?check_run_id=26906365350

Upstream

Issue: Mergifyio/mergify#5075
Discussion: Mergifyio/mergify#5117