dev-cmd/bump: add --no-fork switch

[p-linnane]

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This PR brings the --no-fork switch in bump-cask-pr and bump-formula-pr to brew bump. Given that maintainers are now using branches within the main repo, this will make things easier in two ways:

1. Maintainers can make manual changes to formulae or casks, save them, and then run brew bump --open-pr --no-fork foo to handle the rest of the PR process with their local changes incorporated.
2. We can use this for BrewTestBot's autobump PR's. This will reduce the thousands of stale branches in the BrewTestBot forks (which I still plan to deal with).

Example PR opened using this branch: Homebrew/homebrew-core#163948

As usual, thanks to @Bo98 for his guidance so I could put this together.

[MikeMcQuaid]

Thanks @p-linnane! Wasn't worth blocking this PR but I think ideally this would intelligently figure out whether to use a fork or not based on maintainer permissions. I'll maybe look at that in future 👍🏻

[p-linnane]

That sounds awesome. It's a bit out of my ability I think, so looking forward to seeing if you take a look!