Create /opt/sysroot via tmpfiles

Add a tmpfile snippet to create the /opt/sysroot directory. Owned by
root:staff and group writable such that the default user can write to
it, making ade sysroot install usable as a user directly without
requiring sudo.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D5248