Update cachix/install-nix-action digest to a809471

Author: renovate[bot]

.github/workflows/nix.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       # Install Nix on the runner
-      - uses: cachix/install-nix-action@56a7bb7b56d9a92d4fd1bc05758de7eea4a370a8 # v31
+      - uses: cachix/install-nix-action@a809471b5c7c913aa67bec8f459a11a0decc3fce # v31
         with:
           nix_path: nixpkgs=channel:nixos-unstable
       # Pull from the cachix cache