Use slices.Sorted(maps.Keys(m)) to get a sorted slices of the keys in
a map.

Do not change packages built during bootstrap, as the bootstrap compiler
(currently 1.22.6) does not have the required maps and slices functions.

Change-Id: Ie35565d241fa14aca56b730a69af010127b659ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/617356


Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>