net/netip: fix formatting of IPv4-in-6 address with zone

Weird, but don't drop the zone when stringifying.

Fixes #50111

Change-Id: I5fbccdfedcdc77a77ee6bafc8d82b8ec8ec7220c
Reviewed-on: https://go-review.googlesource.com/c/go/+/371094


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Trust: Matt Layher <mdlayher@gmail.com>
Trust: Ian Lance Taylor <iant@golang.org>