Maybe fix flatpak build checkout failures

[oskirby]

Description

I have noticed a lot of Flatpak build failures lately, which seem to end with a failure to checkout the Mozilla VPN sources, but the build never seems to fail on the main branch. I suspect that this is due to an inability to find the merge commit SHA in whatever build worker is running the build. We might have better luck using github.event.pull_request.head.sha instead, which is the latest commit on the PR branch rather than the merge commit.

Reference

i.e Jira or Github issue URL

Checklist

• My code follows the style guidelines for this project
• I have not added any packages that contain high risk or unknown licenses (GPL, LGPL, MPL, etc. consult with DevOps if in question)
• I have performed a self review of my own code
• I have commented my code PARTICULARLY in hard to understand areas
• I have added thorough tests where needed