Revert "Use nixpkgs master for now"

This reverts commit d8c2ba8.

No reason we can't presently switch back to the unstable branch.

Author: leungbk

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ jobs:
     - name: Install Nix
       uses: cachix/install-nix-action@v13
       with:
-        nix_path: nixpkgs=https://github.com/NixOS/nixpkgs/archive/master.tar.gz
+        nix_path: nixpkgs=channel:nixos-unstable
 
     - name: Add nix-community cache
       uses: cachix/cachix-action@v10