Unfortunately what I've learned is that Fenix builds are not being uploaded to the same places that Fennec builds were so that we can link to them easily. They do show up in GitHub releases as you said, but those are manually managed and unreliable at this time. For now our plan is outlined in this issue:
Basically we plan to remove the Android options from the form on /all and add Play Store and App Store links for the Android and iOS browsers respectively. It's unfortunate that we'll no longer be offering direct APK downloads on the site. Hopefully the team will make the builds available in an automated and reliable way soon (see bug 1614763) and we can add those options back to the form.
Thanks very much for filing this bug.