Nixpkgs-update automation and new sources

nixpkgs-update is no longer running on my main desktop PC!

Thanks to the generous help and support of @zimbatm, @adisbladis, and other supporters of the nix-community GitHub organization, I have migrated nixpkgs-update to run on nix-community infrastructure. As part of the move, I’ve configured nixpkgs-update to run continuously: when a run finishes another starts the next morning.

Also, thanks to @Synthetica, we now have a new source of version updates. He wrote nixpkgs-update-github-releases which gathers new versions from GitHub’s releases API. This new source is exciting because it includes more up-to-date versions than Repology.

Thank you to the nixpkgs-update patrons for your support and belief in the project. Your support has both encouraged and inspired my efforts.