TestAmbientCapsUserns also needs to be skipped, e.g. in case the test is
run inside a chroot.

Updates #34015

Change-Id: I53913432fe9408217edfe64619adbfd911a51a7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/196500
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>