Install CACHEDIR.tag in /var/lib/plocate.

This helps certain tools understand that everything in the directory
is autogenerated. Suggested by Marco d'Itri.

mkdir.sh

@@ -1,2 +1,9 @@
 #! /bin/sh
 mkdir -p ${DESTDIR}/"$1"
+
+cat <<EOF >${DESTDIR}/"$1"/CACHEDIR.TAG
+Signature: 8a477f597d28d172789f06886806bc55
+# This file is a cache directory tag created by plocate.
+# For information about cache directory tags, see:
+#	https://bford.info/cachedir/
+EOF