Repository discovery doesn't have the right permission

[marcoieni]

With #2630 we should create PRs when a new repository is added manually to one of the rust-lang organizations.
However, this PR uses the GitHub app of the dry run to open the PR. That github app lacks permissions to open PRs.

Should we create a new GitHub App with write permission on pull request and contents?

[Kobzol]

While allowing the dry run app to open PRs is probably not so terrible, having a separate app for that would be better, yeah.