runtime: keep cleanup closure alive across adding the cleanup special
This is similar to the weak handle bug in #70455. In short, there's a
window where a heap-allocated value is only visible through a special
that has not been made visible to the GC yet.
For #70455.
Change-Id: Ic2bb2c60d422a5bc5dab8d971cfc26ff6d7622bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/630277
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com> Reviewed-by: Carlos Amedee <carlos@golang.org>