Package AppImages for Linux

[ravener]

Implements #415 by introducing AppImages.

This is a rough draft and meant to be a proof of concept, I've implemented it in the ci.yml for the purpose of easier testing instead of needing to trigger a release, but this needs to be moved to the release workflow when ready.

I've created this draft PR to get feedback if anything needs to be done differently, and additional cleanups that may be needed, other than that I can start copying it over to the release workflow and finalize the PR.

So a quick rundown of what is happening for the devs who are unfamiliar with AppImages:

• AppImages needed FUSE so I had to add an additional dependency on the apt install command. This should be available on most distros preinstalled but it wasn't in our CI environment so I added that. See https://docs.appimage.org/user-guide/troubleshooting/fuse.html
• The idea is we create an AppDir which holds like a custom prefix with the /usr folders and what not, see the full spec at https://docs.appimage.org/reference/appdir.html
• within the AppDir we need a file AppRun this is responsible to prepare the environment and startup the app, this can be a shell script or anything if need be but the common convention is to use these prebuilt AppRun binaries by the AppImage team themselves. These setup the environment to start it from the .desktop file.
• I created a minimal .desktop file and chose the icon that was available in assets/app_icon.png hope that is fine. the .desktop can be extended further if anything needs to be added, like a comment (program description basically)
• The AppImageKit provides us a handy tool to turn the AppDir folder into the final .AppImage and so once we finished creating the final directory structure I create the AppImage with it and upload the artifact. I chose to download the continous version because latest release is like from 2020 and very outdated like it won't accept versions newer than 1.0 for the freedesktop .desktop file for example.
• Seems like the flutter bundle wasn't meant to work in an absolute unix filesystem structure so putting the lib in /usr/lib in the appdir did not work, so I chose to stuff it all in the bin so they are relative to the binary, this should be fine as I've seen other appimages stuff everything in there as well.
• This bundles whatever flutter gave us but not the whole libraries, things like gtk should exist on the user's system, so it's not fully contained but it's the same assumption as downloading the .tar.gz and I believe is fine.

To test this you can download the artifact from the latest commit in my fork unzip the artifact and chmod +x liftoff.AppImage and finally ./liftoff.AppImage

TODO

• Move the workflow to the release pipeline instead
• Potentially also implement this update information https://docs.appimage.org/packaging-guide/optional/updates.html

Like I said so far I'm opening this as a draft PR to get some feedback, once I get the OK, I will squash my commits and any other cleanups suggested.

[JakobDev]

See #236

[ravener]

I'm not sure what I can do?

[JakobDev]

Just wait until the other PR is merged