Support running autopkgtest with missing home directory and with http_proxy set

The source autopkgtest fails in case the home directory does not exist: maven could not transfer the downloaded artifact.

Use a temporary directory as home directory.

The Ubuntu autopkgtest infrastructure uses a HTTP proxy to access the Internet.

Copy the http_proxy/https_proxy environment variables into the Maven settings.xml file to make Maven use the proxy.

LP: #2115740