prepare-volatile-img.sh: create volatile.img group accessible

Otherwise qvm-create-default-dvm may fail to include it in
saved-cows.tar, which will lead to DispVM being not really disposable.

Fixes QubesOS/qubes-issues#2200

linux/aux-tools/prepare-volatile-img.sh

@@ -20,5 +20,6 @@ if [ -e "$FILENAME" ]; then
 	exit 1
 fi
 
+umask 002
 TOTAL_SIZE=$[ $ROOT_SIZE + $SWAP_SIZE + 512 ]
 truncate -s ${TOTAL_SIZE}M "$FILENAME"