dnsdist: Disable the send wrappers in our CI

dnsdist: Disable the send wrappers in our CI

The way the send wrappers are implemented, reading the data _after_
it has been sent, cause them to report a data race that does not
exist with existing implementations:
- we call `send()` from thread 1 to send a query to a backend, never
  touching the data or associated metadata again from that thread
- we get a response from the backend in a different thread, thread 2,
  which will then access the metadata and sometimes (truncated UDP
  answers following a DoH query) even modify the data itself
- ASAN and TSAN complain because the wrapper might still be reading
  the data after the UDP datagram has been sent, which is effectively
  a race, but it does not really make any sense for an actual
  implementation of `send()` to do that.

We work around that by disabling the `send()` wrappers in our CI,
for the dnsdist regression tests only, via `intercept_send=0`.