The upgrade apparently made it necessary to also add `XDG_DATA_DIRS` to fix the crash mentioned in the added comment. I copied this solution from: `pkgs/applications/misc/devdocs-desktop/default.nix`
This uses the .AppImage binary release, until we have a real source package (see https://github.com/marktext/marktext/issues/1647). I used the `notable` package as a template.