Fix a6c042855e: [CI] Deployment creation was failing

[glx22]

Motivation / Problem

My testing in glx22#32 had a flaw, using github.event.workflow_run.head_branch was fine because the test branch is in my repo.
But that doesn't work when the branch is in another repo like for #14828 or #15391.
I was able to test this case in glx22#41 and see the API reply which confirmed the issue "message": "No ref found for: glx_test".

Description

Don't use the head branch to refer to a PR, but instead use the head sha.
glx22#41 shows the deployment is properly created and updated once publish step is done, and it's not a same repo PR this time :).

Publishing fails due to missing credentials, but it's properly linked to the PR.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker