cmd/go: add a GODEBUG to limit the number of concurrent network connections

cmd/go: add a GODEBUG to limit the number of concurrent network connections

I implemented this in order to debug connection failures on a
new-to-me VM development environment that uses Cloud NAT. It doesn't
directly fix the bug, but perhaps folks will find it useful to
diagnose port-exhaustion-related flakiness in other environments.

For #52545.

Change-Id: Icd3f13dcf62e718560c4f4a965a4df7c1bd785ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/473277
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>