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

Currently, rtcwake -m off doesn't work at all, since /sbin/shutdown is missing. Creating a circular dependency on its provider (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.