internal/runtime/maps: store group across Iter.Next calls

internal/runtime/maps: store group across Iter.Next calls

A previous CL kept it across loop iterations, but those are more rare
than call iterations.

For #54766.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-swissmap
Change-Id: Ieea0f1677e357f5e451650b1c697da7f63f3bca1
Reviewed-on: https://go-review.googlesource.com/c/go/+/621116
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>