borg-client: use the correct base directory to exclude files

roles/borg-client/templates/borg-backup.sh.j2

@@ -56,10 +56,10 @@ borg create -v --stats -C lz4 \
     -e /var/cache \
     -e /var/lib/archbuild \
     -e /var/lib/archbuilddest \
-    -e "$snapshotdir/var/cache" \
-    -e "$snapshotdir/var/lib/archbuild" \
-    -e "$snapshotdir/var/lib/archbuilddest" \
-    -e "$snapshotdir/srv/archive" \
+    -e "$backup_mountdir/var/cache" \
+    -e "$backup_mountdir/var/lib/archbuild" \
+    -e "$backup_mountdir/var/lib/archbuilddest" \
+    -e "$backup_mountdir/srv/archive" \
     {{ backup_host }}:{{ backup_dir }}::$(date "+%Y%m%d-%H%M%S") "$backup_mountdir"
 borg prune -v {{ backup_host }}:{{ backup_dir }} --keep-daily=7 --keep-weekly=4 --keep-monthly=6