ci: use nixpkgs-unstable channel instead of a pinned one

No need to pin this to a specific release, it just creates more confusion.

Author: phip1611

.github/workflows/developer_productivity.yml

@@ -37,7 +37,7 @@ jobs:
         with:
           # This channel is only required to invoke "nix-shell".
           # Everything inside that nix-shell will use a pinned version of nixpkgs.
-          nix_path: nixpkgs=channel:nixos-23.05
+          nix_path: nixpkgs=channel:nixpkgs-unstable
       # Dedicated step to separate all the
       # "copying path '/nix/store/...' from 'https://cache.nixos.org'."
       # messages from the actual build output. This job takes ~60secs.