ddnet-server: build fixes (#391366)

pkgs/by-name/dd/ddnet/package.nix

@@ -111,7 +111,7 @@ stdenv.mkDerivation rec {
     rm -rf $out/share/metainfo
   '';
 
-  preFixup = lib.optionalString stdenv.hostPlatform.isDarwin ''
+  preFixup = lib.optionalString (stdenv.hostPlatform.isDarwin && buildClient) ''
     # Upstream links against <prefix>/lib while it installs this library in <prefix>/lib/ddnet
     install_name_tool -change "$out/lib/libsteam_api.dylib" "$out/lib/ddnet/libsteam_api.dylib" "$out/bin/DDNet"
   '';
@@ -137,6 +137,6 @@ stdenv.mkDerivation rec {
       Scrumplex
       sirseruju
     ];
-    mainProgram = "DDNet";
+    mainProgram = "DDNet${lib.optionalString (!buildClient) "-Server"}";
   };
 }