lepton: only build on x86_64-linux for now

Darwin and `aarch64-linux` builds currently fail[1][2].

Also cleaned up the `meta` section a bit.

[1] https://github.com/NixOS/nixpkgs/pull/56108/checks?check_run_id=66442317
[2] https://github.com/NixOS/nixpkgs/pull/56108#issue-254801596