[dev.typeparams] runtime: mark TestGcSys as flaky

I don't know what this test is doing, but it very frequently flakes
for me while testing mundane compiler CLs. According to the issue log,
it's been flaky for ~3 years.

Updates #37331.

Change-Id: I81c43ad646ee12d4c6561290a54e4bf637695bc6
Reviewed-on: https://go-review.googlesource.com/c/go/+/336349


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>