Bundle appimagetool properly

[probonopd]

Looks like appimagetool nowadays has dependencies that also need to be deployed.

@TheAssassin says we should do something like

• https://github.com/AppImage/AppImageKit/blame/master/ci/build-appdir.sh#L61-L71
• https://github.com/linuxdeploy/linuxdeploy-plugin-appimage/blob/master/ci/build-appimage.sh#L50-L58

Needed for:

• Add partial Qt 6 support. #541

[probonopd]

Just a reminder that this is still broken.
The mistake may be mine but currently I don't have the time to dig into this.