feat(nix): more `maxJobs` and CPU cores for `linux-builder`

Signed-off-by: Seongmin Lee <[e-mail]>

nix/darwin.nix

···
   nix.settings.trusted-users = [ "@admin" ];
   nix.settings.extra-trusted-users = [ "boltless" ];
 
-  nix.linux-builder.enable = true;
+  nix.linux-builder = {
+    enable = true;
+    maxJobs = 4;
+    config.virtualisation = {
+      cores = 6;
+    };
+  };
 
   # Enable alternative shell support in nix-darwin.
   # programs.fish.enable = true;