Add support to test Flatpak with PyGame

[rmartin16]

Previously, the app verification job for Flatpak would only run for Toga; this allows it to run for PyGame as well.

Alternatively, to avoid this in the future, it may be better to remove this check against the "framework" altogether and instead require the repo running the job to exclude invalid frameworks.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

All makes sense - and I agree it might be better to make the default "run everywhere", and then make the individual users of the template filter the specific frameworks that are tested.

[rmartin16]

All makes sense - and I agree it might be better to make the default "run everywhere", and then make the individual users of the template filter the specific frameworks that are tested.

Just fyi....this actually may be a bit more involved than I initially thought.

Presently, the primary consideration for whether a build is attempted for a particular output format is the OS of the GitHub CI runner. And then only Toga is allowed for the mobile platforms and PySide2 is excluded for several.

So, I think app-build-verify.yml may require a new input to explicitly specify the output formats to test for the a framework....instead of the inferring it from the runner OS.

[edit]
Though, requiring of specific list of output formats everywhere means that when a new one is added.....any repo using app-build-verify.yml will need to be updated individually.......maybe the output formats can be grouped (e.g. desktop-formats and mobile-formats or something)....idk...more thought needed