dracut: Support /usr/bin as 'systemctl' path

On openSUSE the initrd has systemctl in /usr/bin, check this path as
well.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Lorenz Hüdepohl <dev@stellardeath.org>
Closes #11487