mkosi: support the case /sbin/init is an absolute symbolic link

In that case, the link points to the host file, thus we cannot update
the file.

mkosi/mkosi.images/minimal-base/mkosi.postinst

@@ -3,6 +3,8 @@ set -e
 
 # We don't use mkosi.extra because /usr/sbin could be a symlink and cp doesn't handle that properly until
 # coreutils 9.5 or newer.
+mkdir -p "$BUILDROOT/sbin"
+rm -f "$BUILDROOT/sbin/init"
 cat >"$BUILDROOT/sbin/init" <<EOF
 #!/bin/bash
 echo "Hello from dummy init, beautiful day, innit?"