Subsitute /run/current-system/sw/bin for hard-coded /{,s}bin

A working /sbin/shutdown (usually provided by systemd) is required by
"rtcwake -m off". Creating a circular dependency on systemd for this
just isn't worth the trouble, so take the straightforward way out.

One could easily make the argument that rtcwake wants to shut down the
*currently running system*, and that the correct API for that *is* in
fact /run/current-system. And it makes a very tempting sort of sense.