cifs-utils: hard-code path '/run/current-system/sw/bin/systemd-ask-password' to fix issue #353 some more

It turns out that mount.cifs is run in a shell environment that doesn't have
$PATH. To find that program, we must commit to some location. The path I used will
work fine on NixOS, but of course it won't work on other Linux distributions. I'm
not sure whether that's an important issue or not.