Build app as a JPackage app-image before making other installers [1 XMR]
Right now, the Linux CI builder uses JPackage to build for each platform, but the package is reset every time. JPackage supports building into an intermediary app-image (not an [AppImage](https://appimage.org/)) and building the individual installers on top of that. Using this instead of rebuilding that will save a lot of CI minutes, and may be necessary once we also have Flatpaks and AppImages.

I've been thinking of doing this for a bit, and @\TheTollingBell raised the idea of putting it in an issue. I'll try and get this done after we merge these two PRs, as that'll be less likely to break anything.
