.github: remove nix channel, is fetched explicitly

.github/workflows/ci.yml

@@ -16,6 +16,4 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - uses: cachix/install-nix-action@v12
-        with:
-          nix_path: nixpkgs=channel:nixos-20.09
       - run: ./test.sh

fetch.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env nix-shell
-#!nix-shell -i python3 -p python3 curl nix
+#!nix-shell -I nixpkgs=./nixpkgs.nix -i python3 -p python3 curl nix
 
 import json
 import re