This ensures that the new `partscan` feature in osbuild works. By
default the containers only have a static snapshot of /dev on a
tmpfs. This means that anything later added by losetup will be
missing inside the container.
It also means that https://github.com/osbuild/osbuild/pull/1468
can be reverted.
In most cases e.g. we do want to show stdout/stderr, and
it's handy to have a debug log when we're running
a subprocess.
While we're here, switch to just forking `cp` in
the setup code.
Signed-off-by: Colin Walters <walters@verbum.org>
