feat: add flatpak builds to the ci for testing

[DioEgizio]

closes #62

[Scrumplex]

I don't really like that we would be having duplicate code here and two places that would require updates

[DioEgizio]

I mean this is only on debug builds (yes I noticed it's building on release too, I have to fix later)
I don't think we can reuse the same yml everywhere

[guihkx]

I think there's an alternative to maintaining two Flatpak manifests (it's a bit hacky, though):

1. Clone org.prismlauncher.PrismLauncher
2. Use yq to update the required bits of the manifest, something like:

- name: Checkout Flatpak manifest
  uses: actions/checkout@v3
  with:
    repository: flathub/org.prismlauncher.PrismLauncher

- name: Install yq
  uses: mikefarah/yq@v4.30.4

- name: Update Flatpak manifest to build using this repo
  run: yq -i '(.modules[] | select(.sources[].url == "*/PrismLauncher/*")).sources = {"type": "dir", "path": ".."}' org.prismlauncher.PrismLauncher.yml

- name: Show modified manifest
  run: cat org.prismlauncher.PrismLauncher.yml

  # build flatpak...

[DioEgizio]

Very hacky and there can be breaking changes related to the build system between releases

[guihkx]

Yeah, it can get tricky... Perhaps another alternative is maintaining a single Flatpak manifest in-tree, like OBS Studio and Firefox both do for their apps:

• https://github.com/obsproject/obs-studio/blob/master/CI/flatpak/com.obsproject.Studio.json
• https://hg.mozilla.org/releases/mozilla-release/file/tip/taskcluster/docker/firefox-flatpak/runme.sh

I'm not sure if this requires a special process within Flathub, though.

[DioEgizio]

i think it's better to keep duplicated for now

[Scrumplex]

Who is going to keep this manifest up-to-date though?

[DioEgizio]

I guess we can update when it's needed, I don't think it'll need frequent updating anyways, and it can be useful for the actual stable flatpak to be able to have the manifest updated and see if there are any regressions