Fix the chroot step introduced in my previous commit

${ROOT?} does not make sense within a chroot, was causing the build to fail

raspi3.yaml

@@ -128,7 +128,7 @@ steps:
   # the partition labeled raspiroot instead of forcing it to mmcblk0p2
   - chroot: root-fs
     shell: |
-      sed -i 's/.dev.mmcblk0p2/LABEL=raspiroot/' ${ROOT?}/boot/firmware/cmdline.txt
+      sed -i 's/.dev.mmcblk0p2/LABEL=raspiroot/' /boot/firmware/cmdline.txt
 
   # TODO(https://github.com/larswirzenius/vmdb2/issues/24): remove once vmdb
   # clears /etc/resolv.conf on its own.