workflow: When updating the issueXX branch, use force push

Otherwise if you try to update the branch with a new /cherry-pick
from the same issue you will run into problems similar as to the
one shown in this workflow:
https://github.com/llvm/llvm-project/runs/5864672298?check_suite_focus=true

Reviewed By: tstellar

Differential Revision: https://reviews.llvm.org/D123365