Attempt to auto-update flatpak crates

[oskirby]

Description

After my last messing around with the Flatpak manifests, it turns out that it wasn't a good idea to check in a static version of flatpak-vpn-crates.json since our Cargo.lock is updated automatically by Dependabot and the two files will quickly fall out of sync. Let's make another attempt to write some automation to ensure it's updated.

The last time I tried this, it got stuck because Github prevents workflows from retriggering themselves (this runs the risk of creating an infinite loop). So we would wind up with a PR that would automatically update the flatpak manifests, but then none of the required checks wound run, leaving the PR in an un-mergeable state. However, there are some recommended workarounds that we can use.

To try and mitigate the risk of an infinite loop, we set up this workflow so that it only runs on dependabot PRs, and only when there are no changes to the linux/flatpak directory.

Reference

Github issue #9895
My last attempt: #9616

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

[mcleinman]

Looks good.