Default CI_SHA/GITHUB_SHA is the sha of _expect-to-be_ merge/squash/etc commit of the pull requests,
and it can be very confusing. If you mark you artifacts with it, its very hard to trace back what was the actual commit.
CI_PR_SHA is the way I solved it for my use cases and I can see im not the first one with this problem.
Fixes#30
Right now the closest this action offers `CI_REF_NAME_SLUG` which is `42-merge`, but its not clean and more importantly cannot be used in numerical contexts which would benefit from having that as `42`. if current event is not a pull-request, it should be empty