mkosi: make sysvinit path inference consistent

The integration tests use /etc/rc.d/init.d if it exists
or falls back to /etc/init.d,
while the mkosi.build.chroot script dereferenced /etc/init.d.

This produces inconsistent results, as sometimes an image can be made
that has systemd built to expect /etc/init.d but /etc/rc.d/init.d
also exists.

mkosi.images/base/mkosi.build.chroot

@@ -44,7 +44,7 @@ EOF
 fi
 
 if [ ! -f "$BUILDDIR"/build.ninja ]; then
-    sysvinit_path=$(realpath /etc/init.d)
+    [[ -d /etc/rc.d/init.d ]] && sysvinit_path="/etc/rc.d/init.d" || sysvinit_path="/etc/init.d"
 
     if [ "$ID" = "centos" ] && [ "$VERSION" = "8" ]; then
         UKIFY="disabled"