build-podman: Don't assume that host is reachable as 10.0.2.2

Simon McVittierequested to merge
wip/smcv/podman-proxy into master

This was true with slirp4netns in its default configuration, but is no longer true when using passt. If the auto-detected proxy is localhost or 127.0.0.x, ask the user to set a suitable proxy via --apt-proxy instead.

Closes: #1078076

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