humility: don't build xtask

pkgs/by-name/hu/humility/package.nix

@@ -30,6 +30,9 @@ rustPlatform.buildRustPackage {
 
   cargoHash = "sha256-GZkHPoDKiqTVwRAWXXbELXC1I/KRO+9sshY8/rGbA4A=";
 
+  # Prevent building and installing xtask, as it's a developer utility not intended for the end user
+  cargoBuildFlags = [ "-p humility-cli" ];
+
   meta = with lib; {
     description = "Debugger for Hubris";
     mainProgram = "humility";