Use nixpkgs master for now

It should unblock updates.

Author: adisbladis

.github/workflows/ci.yml

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