workflows/sponsors-maintainers-man-completions: only run on branches

This prevents running the jobs on tags. It can be dangerous to
force-push a tag, even though previous workflow runs actually failed.

Example of previous failure:
https://github.com/Homebrew/brew/actions/runs/4657211778/jobs/8241583576

Signed-off-by: Ruoyu Zhong <zhongruoyu@outlook.com>