Use internal resolver for default bridge

[robmry]

Description

Related to:

• Add resolver for default bridge, remove default nameservers #47602
• Revert internal resolver for default bridge #48020

#47602 set up the internal resolver for the default bridge, removing the need for checking whether systemd's resolver is running (to find its upstream requests), making it possible to use an IPv6 upstream server from an IPv4 network, removing the need for fallback servers when the host's DNS servers aren't reachavle from the container - and generally making the default bridge more like a custom bridge.

That change was reverted in #48020 because buildkit doesn't run an internal resolver (or DNS forwarder), so containerised buildkit could end up using the Google fallback servers, when it would previously have used a local DNS server.

We need to come up with a plan for restoring the change, and do that.