Add `skip_setsid` option to nsjail for sbox.

ABFS project uses the process group to trace all the
file-system activities of a given build command.
The assumption is that the build commands do not change the
process group, otherwise some of the file operations can be
missed.

Bug: 376530561
Change-Id: Iff32df924e1cb20be2ce4dfe3355d833c0fb9e8e