test: mark test-child-process-fork-net as flaky

`flaky-test-child-process-fork-net` has been failing constantly for the
past few days, and all solutions suggestes so far were didn't work.
Marking it as faky while the issue is not fixed.

Ref: #21012
Ref: #20973
Ref: #20973

PR-URL: #21018
Refs: #21012
Refs: #20973
Refs: #20973
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

test/parallel/parallel.status

@@ -9,6 +9,7 @@ prefix parallel
 test-postmortem-metadata: PASS,FLAKY
 
 [$system==win32]
+test-child-process-fork-net: PASS,FLAKY
 
 [$system==linux]