feat: linux builds

[FoxtrotSierra6829]

Fixes #514

Summary of Changes

Creates linux builds

• AppImage
• RHEL/Fedora based distributions (.rpm)
• Debian based distributions (.deb)
• Snap
• Flatpak

Screenshots (if necessary)

Additional context

• The additional electron-builder configs are necessary because of course not all package formats like to be configured the same way, especially a thing as simple as a desktop icon has proven to be as unstandardized to configure as one can make it.
• Since the .AppImage was bigger than 100MiB I had to change the Cloudflare R2 upload to use rclone instead of our worker. As a byproduct, it's faster and easier to maintain. See ci: direct cdn upload #538 which should be merged first
• GitHub vmaster and the CDNs hold the packages without version number to maintain fixed URLs. The GitHub releases include the version number. The .exe has also been renamed to match the common naming format.
• The Windows and AppImage versions auto-update. For other Linux packages, this may technically be possible but not intended (.deb / .rpm - requires sudo) or not possible at all. The functionality of the auto-update has been tested by me.

Discord username (if different from GitHub): foxtrotsierra

[alepouna]

Need to take a closer look @ the workflows & the autoupdater, but here's some comments so far.

[6639835]

lgtm