Upgrade nixpkgs to 20.09, rules_haskell to 0.13, rules_nixpkgs to 0.7#2

avdv wants to merge 2 commits intoTreeTide:masterTreeTide/haskell-bazel-nix-template:masterfrom

avdv:upgrade-nix-and-bazel-rulesavdv/haskell-bazel-nix-template:upgrade-nix-and-bazel-rulesCopy head branch name to clipboard