relay: Look at the omit IPv6 flag when publishing

In two instances we must look at this flag:

1. When we build the descriptor so the IPv6 is NOT added to the descriptor in
   case we judge that we need to omit the address but still publish.

2. When we are deciding if the descriptor is publishable. This flags tells us
   that the IPv6 was not found reachable but we should still publish.

Fixes #40279

Signed-off-by: David Goulet <dgoulet@torproject.org>