Review shell scripts

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

hosts/common/configs/system/impermanence/scripts/wipe.sh

@@ -17,11 +17,11 @@ mount "/dev/mapper/$DEVICE" /mnt/btrfs
 if [[ -e /mnt/btrfs/@ ]]; then
   mkdir -p /mnt/btrfs/@.bak
   timestamp=$(date --date="@$(stat -c %Y /mnt/btrfs/@)" "+%Y-%m-%d_%H:%M:%S")
-  mv /mnt/btrfs/@ "/mnt/btrfs/@.bak/${timestamp}"
+  mv /mnt/btrfs/@ "/mnt/btrfs/@.bak/$timestamp"
 fi
 
 find /mnt/btrfs/@.bak/ -maxdepth 1 -mtime +14 | while IFS= read -r i; do
-  delete_subvolume_recursively "${i}"
+  delete_subvolume_recursively "$i"
 done
 
 btrfs subvolume create /mnt/btrfs/@