only write fleek package once per arch

[bketelsen]

[i] Applying configuration
error: attribute 'packages.aarch64-darwin.fleek' already defined at /nix/store/9bqdpl9xzy2ah8m4gwhgc3w9byaxipml-source/flake.nix:23:6

   at /nix/store/9bqdpl9xzy2ah8m4gwhgc3w9byaxipml-source/flake.nix:27:6:

       26|
       27|      packages.aarch64-darwin.fleek = fleek.packages.aarch64-darwin.default;
         |      ^
       28|

[bketelsen]

fixed by #284