(#16451) Use PATH to find systemctl for systemd service provider

    Without this patch Puppet has a difficult time locating the systemctl
    executable in recent versions of Archlinux.  This problem is caused by
    the executable moving from /bin/systemctl to /usr/bin/systemctl.

    This patch addresses the problem by eliminating the fully qualified path
    and instead relying on the PATH environment variable to locate the
    command.