[gardening] Exclude standalone/io/secure_builtin_roots_test from isolate stress testing

This test is unreliable and sometimes fails with errors

  HttpException: Connection closed before full header was received, uri = https://www.google.com,

or

  Expect.equals(expected: <fail>, actual: <pass>, 'Unexpected failed connection') fails.

Change-Id: I3b21c2014756e5b6c90509b2cae3e62548f90ff2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/436940
Auto-Submit: Alexander Markov <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: alexmarkov

runtime/tests/concurrency/stress_test_list.json

@@ -3515,7 +3515,6 @@
   "../../../tests/standalone/io/regress_50206_test.dart",
   "../../../tests/standalone/io/regress_8828_test.dart",
   "../../../tests/standalone/io/regress_9194_test.dart",
-  "../../../tests/standalone/io/secure_builtin_roots_test.dart",
   "../../../tests/standalone/io/secure_socket_argument_test.dart",
   "../../../tests/standalone/io/server_socket_close_listen_test.dart",
   "../../../tests/standalone/io/server_socket_exception_test.dart",