Some modules support configuration of http(s) proxies today (e.g. ant, maven, nodejs) - but they are only honoured if building with host provider and the environment variables are already in the environment. Add two explicit build options for http_proxy and https_proxy, with matching environment variables to support typical application behavior. Only add these to the environment if provider is one of 'lxd', 'multipass', 'host', or 'managed-host'. Otherwise, either: (a) warn if found in the user's environment. (b) error if not found in environment, and therefor must have been configured with an explicit command-line option. Add tests for coverage of get_build_environment_flags() with new http_proxy and https_proxy handling.