appimage: package QT platforms

[GitMensch]

[...] maybe all you need to do instead is run hotspot -platform offscreen ... or QT_QPA_PLATFORM=offscreen hotspot ... instead?

[...] the first test did not worked out

qt.qpa.plugin: Could not find the Qt platform plugin "offscreen" in ""

This is with the appimage I guess? Packaging problem - could be solved there.

Originally posted by @milianw in https://github.com/KDAB/hotspot/issues/549#issuecomment-1803391337