Require libknot 2.6.4 to avoid mysterious problems with DNS-over-TLS.

Respdiff on kresd under load showed that DNS-over-TLS has higher ratio
of SERVFAILs than other transports. For some reason the problem
disappeared after upgrading from libknot 2.6.3 to 2.6.4, and appeared
again after downgrade.
15 jobs for libknot
in 10 minutes and 15 seconds and was queued for 14 seconds