test: mark test-child-process-execsync flaky on AIX

Refs: https://github.com/nodejs/node/issues/24921

PR-URL: https://github.com/nodejs/node/pull/25031


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Bradley Farias <bradley.meck@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>