build-rootfs: Pass config path to ldconfig

`ldconfig` has started complaining that it needs a configuration file
when using the `-r` argument:

> /mnt/gentoo/usr/sbin/ldconfig: need absolute file name for
> configuration file when using -r

build-rootfs.sh

@@ -76,7 +76,10 @@ if [ -f /mnt/gentoo/etc/udev/hwdb.bin ]; then
     mv /mnt/gentoo/etc/udev/hwdb.bin /mnt/gentoo/usr/lib/udev/
 fi
 
-qemu-${target%%-*} /mnt/gentoo/usr/sbin/ldconfig -r /mnt/gentoo
+qemu-${target%%-*} \
+/mnt/gentoo/usr/sbin/ldconfig \
+    -r /mnt/gentoo \
+    -f /mnt/gentoo/etc/ld.so.conf
 
 rm -f /mnt/gentoo/lib/tmpfiles.d/provision.conf
 systemd-tmpfiles --root=/mnt/gentoo -E --exclude-prefix=/var --create