tmpfiles: create cache and use proper tmpfiles name

systemd/tmpfiles/knot-resolver.conf

 # tmpfiles.d(5) runtime directory for knot-resolver (kresd)
 #Type Path                            Mode UID           GID          Age Argument
     d /run/knot-resolver              0751 root          root          -   -
+    d /var/cache/knot-resolver        0750 knot-resolver knot-resolver -   -