Override previous NayuOS build image

If two NayuOS images are built on the same day, the second one will not get zipped. The -f flag in gzip can be used to overwrite the previous image

/reviewed-on nexedi/slapos!336

software/nayuos/scripts/cros_full_build.in

@@ -356,7 +356,7 @@ for hashfunction in md5sum sha1sum sha256sum sha512sum; do
 done
 
 for file in $(ls *.img); do
-  gzip -9 ${file}
+  gzip -9 -f ${file}
 done
 
 exit 0