runtime: TestGcSys is still flaky [1.17 backport]

[gopherbot]

@bcmills requested issue #37331 to be considered for backport to the next 1.17 minor release.

@gopherbot, please backport to Go 1.17. This test is skipped on the main branch, but still failing semi-regularly on the release branch.

[gopherbot]

Change https://go.dev/cl/406974 mentions this issue: [release-branch.go1.17] runtime: mark TestGcSys as flaky

[mknyszek]

This is just a testing skip that improves release builder stability.

[gopherbot]

Closed by merging 0e7138a to release-branch.go1.17.