workflows/sponsors-maintainers-man-completions: fix git-try-push failure

[ZhongRuoyu]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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I think this should actually fix the failure seen in https://github.com/Homebrew/brew/actions/runs/4651602426/jobs/8231313421. origin_branch falls back to branch. This means that the git-try-push step will now push to and rebase on the same remote branch. (Previously, it rebased on master, not the branch to push to.)

Note that this also means the branch ${{ steps.update.outputs.branch }} (sponsors-maintainers-man-completions or PR branch) will not be rebased onto origin/master.

[rlane2126]

good

[MikeMcQuaid]

Makes sense, thanks again @ZhongRuoyu!