Make it work on forks instead of the main repos

[f-f]

Right now, the code will create branches locally when doing updates, and then push them as branches to their origin repo.
This is fine for @pacchettibotti because it has write access to all of them, but it's not fine if someone else wants to run the project.

I think the main issue is this line

pacchettibotti/src/PacchettiBotti/Run.hs

Line 98 in 85234a5

, "git push --set-upstream origin " <> branchName

..where we set origin as the upstream. I suspect that's not the only place where this idea is hardcoded though.

So we should extend the code so that:

• we can specify a username in the config.json or with an envvar
• ..and then when pushing to upstream, we'd check if that username has a fork of the repo, and if not we'd create it and push to it.