Wow, that is a tricky one! I chatted with @ebolyen a bit about this today - we both think that there is some piece of network infrastructure sitting between your computer and the bioconductor servers that is terminating the connection prematurely (maybe an institutional firewall?). This is a bummer, because there probably isn't much that can be done in that department.
We don't develop conda, so we probably don't the complete picture of how this could be accomplished, but a quick Google search shows you might be able stick that file in the pkgs dir in your miniconda directory, then you could try experimenting with adding the --use-local, --offline, and --download-only flags to our traditional QIIME 2 native install command. I don't think we have any experience with that here in the QIIME 2 office, so your mileage may vary. Sorry, this is a bummer - we are considering our long-term distribution/installation options, since conda isn't always a viable solution for everyone, sorry you got caught in that! Thanks for your patience, and please keep us posted! 