A workaround I've used sometimes to retrigger CI is to do `git commit --amend --no-edit --reset-author`. This changes the sha by changing the author timestamp. Then you can force push it to the PR branch
$ git init
$ git commit -m hello --allow-empty
[master (root-commit) 1177c71] hello
$ git commit --amend --no-edit --allow-empty
[master (root-commit) 7dcb290] hello