syscall: skip testAmbientCaps tests if there is no nobody user
When there is not a nobody user (for example inside Docker), the
tests TestAmbientCaps and TestAmbientCapsUserns should be skipped
instead of failing.
Fixes #71644
Change-Id: I7f92db19e2b6f449d8d897650a0ecd89f5150f4a
GitHub-Last-Rev:
a4c4f5bb61929b4981dc0b92d773bd4ef13d7d3d
GitHub-Pull-Request: golang/go#71729
Reviewed-on: https://go-review.googlesource.com/c/go/+/649396
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>