Build app as a JPackage app-image before making other installers [0.5 XMR]

[Jabster28]

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) 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.

[Jabster28]

Should've mentioned: blocked by #1222, #1223, and #1250

[github-actions]

There is a bounty on this issue. The amount is in the title. The reward will be awarded to the first person or group of people whose solution is accepted and merged.

In some cases, we may assign the issue to specific contributors. We expect contributors to provide a PR in a reasonable time frame or, in case of an extensive work, updates on their progress. We will unassign the issue if we feel the assignee is not responsive or has abandoned the task.

Read the full conditions and details of our bounty system.

[woodser]

I've added a bounty of 0.5 XMR to this issue.

@TheTollingBell (from your comment).

[TheTollingBell]

I'm on it. Should be done by end of tomorrow EST time.

[TheTollingBell]

Sorry, forgot that #1230 was blocking. I'll wait till all blocking PRs are merged before starting.

[woodser]

@TheTollingBell the changes from #1230 have been merged to master, so we're ready to build to the intermediary JPackage app image.

[woodser]

@Jabster28 has expressed interest in working on this too, in which case we're happy to take the first acceptable solution, in the interest of time for the next release.

[TheTollingBell]

Almost done with my changes. I forgot to check on this issue lol.