Fix create_draft_release when not specifying a specific commit

[andrewlock]

Summary of changes

Add step to calculate sha to use in create_draft_release GH Action

Reason for change

The existing script assumes you will always force a specific commit, but that's not required

Implementation details

Added a small bash script to choose whether to use the "default" sha or the forced one, and reuse the output throughout the script

Test coverage

Did some (safe) test runs to confirm the correct value is used when doing forced version (here) and non-forced version (here)

[pierotibou]

LGTM thanks.
As discussed offline, I think we should remove this from the critical path of the release and do it a posteriori, with a release trigger. We can discuss with R&P if needed