Fixes #214.

• For push events (applies mostly for main), we rely on if: !contains(github.event.commits[0].message, '[skip *]') to skip the job.
• For pull_request events, since the commit messages are not available in github context, we rely on a separate step which gets the commit message, and if it contains [skip *], the workflow is 'cancelled' by exit 1.
• Additionally, I tried to make the Checkout repo/Checkout repo (PR) steps more coherent - in most workflows these steps checkout the head_ref_branch obtained from Get branch names step. This way we ensure that git will always get the commit message from the proper commit.