GitHub Actions: Use latest stable nixpkgs channel

.github/workflows/donate.yml

@@ -19,5 +19,7 @@ jobs:
         run: |
           curl https://nixos.org/nix/install | sh
           . ~/.nix-profile/etc/profile.d/nix.sh
+          # Use latest stable nixpkgs channel
+          nix-channel --add https://nixos.org/channels/nixos-19.09 nixpkgs
           nix-channel --update
           nix run -f $TOOL -c donate-ci