cmd/go/internal/web: improve IP check testing on ipv6 env

The existing implementation lacks consideration of running test on a
machine which has ipv6 address but no ipv4 address. Use net.IP.IsLoopback
and net.IP.IsUnspecified instead of hardcoded addresses.

Fixes: #48575

This PR will be imported into Gerrit with the title and first
comment (this text) used to generate the subject and body of
the Gerrit change.

Change-Id: I9c3c26d2ba13c7a24065751b59a1e002098ed654
GitHub-Last-Rev: fc45adbf7b944122d8f07bd451a8eeed1e69140c
GitHub-Pull-Request: golang/go#48850
Reviewed-on: https://go-review.googlesource.com/c/go/+/354609


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Jay Conrod <jayconrod@google.com>