Use host apt proxy configuration

It would be convenient if VMs started by multipass shared the apt proxy configuration of their (Ubuntu) host.

I've got an apt proxy on my local network, published via avahi, discovered via `Apt::Acquire::AutoProxy`. It would be very convenient for multipass VMs to use it (particularly for the snapcraft VMs)

Start by tracing how Multipass starts VMs and how the host's Apt::Acquire::AutoProxy configuration could be discovered and passed into them. Verify the result with an Ubuntu host using an Avahi-published apt proxy, including the snapcraft VM case, and confirm that the VM uses the host proxy configuration.

Written by the indexing model from the issue text.