build(ci): build flatpak in CI

[FineFindus]

Fixes #599.
Adds a new GitHub action that builds Workbench in the CI. The finished Flatpak is automatically uploaded as an artifact.
As it currently uses the 45beta runtime, the non-beta extensions need to be installed manually.

[sonnyp]

Thanks. I had something in the work for building the Flatpak on CI but this looks good.

You're the 3rd person wanting Flatpak bundles on every commit in main. Can you help me understand why?

I'm a bit uncomfortable rebuilding and bundling everything on every single commit in main, plus it won't update so I'm not sure I see the benefit?

Is it just a quick way to test the dev version?

[FineFindus]

I mainly enabled it for testing and developing the action, which took a few commits. I squashed them so as not to pollute the main branch.

Having a flatpak build of the main branch can be both an effective way to catch build bugs, and a quick way for designers/users to test the new changes without having to compile the application themselves.
I'm also happy to remove it.

[sonnyp]

We're now on GNOME nightly repository \o/

https://blog.sonny.re/workbench-news