Update; also change from
pname = "package-unstable";
version = "YYYY-MM-DD";
to
pname = "package";
version = "unstable-YYYY-MM-DD";
This allows $pname as $repo in fetchFromGitHub and also seems to be the
more common way in nixpkgs.
Linux 5.9.x is in upstream now, thus remove. We continue pointing
linuxPackages_latest to linux_5_9, though, since this still points to
linux_5_8 upstream.