[release-branch.go1.12] net: use network and host as singleflight key during lookupIP

In CL 120215 the cgo resolver was changed to have different logic based
on the network being queried. However, the singleflight cache key wasn't
updated to also include the network. This way it was possible for
concurrent queries to return the result for the wrong network.

This CL changes the key to include both network and host, fixing the
problem.

Fixes #31062

Change-Id: I8b41b0ce1d9a02d18876c43e347654312eba22fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/166037


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit e341bae0)
Reviewed-on: https://go-review.googlesource.com/c/go/+/170320
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>