nix: Add in a postBuild hook pkgs.radicle-node — radicle-desktop

Making sure that when people install radicle-desktop for nix they get also pkgs.radicle-node

         }
         // (if self ? rev || self ? dirtyRev then {
           GIT_HEAD = if self ? rev then self.rev else self.dirtyRev;
         } else {});
         } else {}) // {
           postBuild = ''
             echo ${pkgs.radicle-node} > $out/nix-support
           '';
         };
       };
     }));
 }