Combined freedesktop build step and metadata for AppImageHub and FlatHub

[mcarans]

Images in flatpak folder to be removed after next FlatHub release

[AnotherCommander]

Is there anything we can do with the Arch build failing due to an apparently improper compiler? Change the Arch workflow maybe so that it utilizes the compiler version which used to work until now?

For the rest I don't have any particular comments as it is almost exclusively Linux related.

[mcarans]

@AnotherCommander I have asked on the GNUstep mailing list about the clang 22 problem and raised an issue on the libs-base repo. In the meantime, I think this PR is good for merge as @oocube has tested it with the AppImageHub here:
https://github.com/oocube/appimage.github.io/actions/runs/23216924263/job/67480065499

I will make a fix if needed for Arch when I get a response from the GNUstep devs. It might be possible to make a hack to get Arch to use older clang, but I'd rather wait to see what the GNUstep devs say before attempting it.

[AnotherCommander]

Sounds good. Merging.