nftables: Set meta.mainProgram to "nft"

The nftables package only has one binary, and this enables running it through
`nix run nixpkgs#nftables`.

pkgs/os-specific/linux/nftables/default.nix

···
     license = licenses.gpl2Only;
     platforms = platforms.linux;
     maintainers = with maintainers; [ izorkin ajs124 ];
+    mainProgram = "nft";
   };
 }