syscall: skip some exec tests in container

syscall: skip some exec tests in container

For those tests there won't be enough permissions in containers.
I decided to go this way instead of just skipping os.IsPermission errors because
many of those tests were specifically written to check false positive permission
errors.

Fixes #21379

Change-Id: Ie25e1d6d47f85bb6b570352638440f3ac1e18e03
Reviewed-on: https://go-review.googlesource.com/58170
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>