config/rootfs/debos: fix ext4 corrupt images

Fix ext4 file system image corruption by adding the postprocess
attribute to the steps that move the image file.

Link: #863
Signed-off-by: Denys Fedoryshchenko <nuclearcat@nuclearcat.com>

config/rootfs/debos/rootfs.yaml

@@ -143,9 +143,11 @@ actions:
   - action: run
     description: Move image to output directory
     command: mv ${ARTIFACTDIR}/rootfs.ext4 ${ARTIFACTDIR}/{{ $basename -}}/
+    postprocess: true
 
   - action: run
     command: xz -f ${ARTIFACTDIR}/{{ $basename -}}/rootfs.ext4
+    postprocess: true
 
   - action: run
     description: update-initramfs step