agent-browser: run tests sequentially to fix flakiness

Several tests share global state (env vars, files on disk) and race
when cargo runs them in parallel. Skipping individual tests is not
enough, so disable parallel test execution instead.

Hydra failure: https://hydra.nixos.org/build/345981617

pkgs/by-name/ag/agent-browser/package.nix

@@ -103,12 +103,8 @@ rustPlatform.buildRustPackage (finalAttrs: {
     writableTmpDirAsHomeHook
   ];
 
-  # Flaky test: reads the AGENT_BROWSER_CDP env variable without using the
-  # shared test lock.
-  checkFlags = [
-    "--skip"
-    "native::actions::tests::test_execute_unknown_command"
-  ];
+  # Some tests use a shared state that makes them flaky when run concurrently.
+  dontUseCargoParallelTests = true;
 
   __darwinAllowLocalNetworking = true;