Make restic cache dir and ensure it's there whenever we invoke it

roles/backup/templates/restic-wrapper.sh

@@ -1,6 +1,10 @@
 #! /bin/sh
 export AWS_ACCESS_KEY_ID="{{ backup_s3_aws_access_key_id }}" 
 export AWS_SECRET_ACCESS_KEY="{{ backup_s3_aws_secret_access_key }}"
+export RESTIC_CACHE_DIR="/var/cache/restic"
+mkdir -p "$RESTIC_CACHE_DIR"
+chown root: "$RESTIC_CACHE_DIR"
+chmod 0700 "$RESTIC_CACHE_DIR"
 exec nice -n 10 restic \
 	-r "s3:{{ backup_s3_aws_endpoint_url }}/{{ backup_s3_bucket }}/restic" \
 	-p /opt/restic-password \