This also bumps the stable nixpkgs to 20.09 as of 2020-11-21, because
there is some breakage in the git build related to the netrc
credentials helper which someone has taken care of in nixpkgs.
The stable channel is not used for anything other than git, so this
should be fine.
Change-Id: I3575a19dab09e1e9556cf8231d717de9890484fb