Always update network interfaces information in cache

[bobrofon]

anet.Interfaces and net.Interfaces use the same zoneCache underneath (and the same lastFetched). But because net.Interfaces is currently not fully functional on Android, it always tries to clear the cache and update the lastFetched timestamp. In certain situations, this may prevent anet.Interfaces from populating the cache at all.

Example pseudocode:

  for {
    _, _ = net.Listen(...) // clears zoneCache, updates the timestamp
    _, _ = anet.Interfaces() // doesn't update the cache because it's up to date
  }

The zoneCache is used to reduce the cost of resolving IPv6 addressing scope zones. Because all expensive operations have already been performed in anet.Interfaces, there is no reason not to update the cache (even if it has recent timestamp).