* appimage.nix -> build-from-appimage.nix to avoid confusion
* .appimage -> x86_64-appimage to allow packaging binaries for architectures
* Pass chromium flag --disable-seccomp-filter-sandbox to the executable
* Use hostPlatform instead of stdenvNoCC.hostPlatform
* Remove unnessesary intermediate variables
they're unnecessary since https://github.com/NixOS/nixpkgs/pull/161739/files
which links the schemas to their correct fhs locations and adds
gsettings-desktop-schemas and hicolor-icon-theme to appimage fhs