community/duo_unix: update source URL to use github

What's different about the source at the URLs?

* New URL: Github is the raw, authoritative source URL.
* Old URL: The vendor takes the Github source,
  runs './bootstrap', and posts it to their download site.

This commit makes the build process more transparent.

Why change the URL now?

The vendor has tagged a new version of duo_unix on github but
has not yet made the new version available at their official
download site.