Fix a misplaced parenthese in serve protocol check
Fix a misplaced parenthese in serve protocol check

This issue made it impossible for clients using a serve protocol of
version <= 2.3 to use the `cmdBuildDerivation` command of servers using
a protocol of version >= 2.6. The faulty version check makes the server
send back build outputs that the client is not expecting.

NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix