Bump cachix/install-nix-action from 30 to 31.

Author: mstange

.github/workflows/nix-packaging.yml

@@ -14,7 +14,7 @@ jobs:
     timeout-minutes: 30
     steps:
       - uses: actions/checkout@v4
-      - uses: cachix/install-nix-action@v30
+      - uses: cachix/install-nix-action@v31
         with:
           nix_path: nixpkgs=channel:nixos-23.11
       # - uses: cachix/cachix-action@v10