mkosi: fix build script to use right sysvinit path

On Fedora /etc/init.d is a symlink to /etc/rc.d/init.d. Our build
scripts default to /etc/init.d since that is the LSB default. Let's make
sure the build script thus follows the symlink correctly and configures
to path explicitly, since otherwise our build artifacts in $DESTDIR are
incompatible with the setup we actually need for Fedora.