delta: explicitly set mainProgram

pkgs/applications/version-management/delta/default.nix

···
     changelog = "https://github.com/dandavison/delta/releases/tag/${version}";
     license = licenses.mit;
     maintainers = with maintainers; [ marsam zowoq SuperSandro2000 figsoda ];
+    mainProgram = "delta";
   };
 }