Jagex-launcher-nix: the official Jagex Launcher (Linux beta) packaged as a flake

Jagex released a native Linux launcher for OSRS. On NixOS it fails out of the box: the “Play Now” button downloads RuneLite.AppImage and executes it, and that doesn’t run here without appimage-run. I packaged it so I could stop using the Bolt launcher. I’ve been working through Vimjoyer’s Nix map and wanted to package something I actually use day to day, so this is my first go at maintaining a package.