Try to phase out GitHub packages list (or give it lesser priority)
See: #1149

GitHub likes to time-out on many ISPs, and if they don't, ISPs like to poke their nose into it. Reliance Jio in India shadow-bans https://raw.githubusercontent.com because "it helps distribute pirated content" and despite a ton of people complaining to them, they just ignore us (they're a big media giant here, so that make sense). 

There's a circumvention by using something like Cloudflare WARP or another VPN. I'd say that we should either phase out the GitHub package list and just run a job on the Nim website server that periodically updates the packages list from the GitHub one (like it currently does!) and remove the GitHub link as a lot of people, including me, have to wait for the GitHub link to time-out, then Nimble advances onto the Nim website mirror. If you CTRL+C during any of that, you end up with what's happening in issue #1149. Or, we can also keep the GitHub link as a fallback in case the Nim website mirror goes down for some reason.

Thoughts?