Update cachix/install-nix-action digest to 7ab6e7f

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@9280e7aca88deada44c930f1e2c78e21c3ae3edd # v31
+      - uses: cachix/install-nix-action@7ab6e7fd29da88e74b1e314a4ae9ac6b5cda3801 # v31
         with:
           nix_path: nixpkgs=channel:nixos-unstable
       # Pull from the cachix cache