Log collection reachability check should use provided proxy.

When providing a proxy via the collect logs UI which is necessary to reach outside of the network, the preflight reachability check prior to initiating the log collection will fail as it does not make use of the provided proxy.

If a proxy is given via the Web UI, we should use that with the preflight check or perhaps provide context that the proxy was not used in this check and allow the user to proceed with the collection.