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.

(I guess one could make the argument that one wants to shut down the
*currently running system*, and that the correct API for that *is* in
fact /run/current-system. It makes a very tempting sort of sense.)