Update cachix/install-nix-action digest to a809471 (#981)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

.github/workflows/check-nix-format.yml

@@ -25,7 +25,7 @@ jobs:
       - name: Calculate changed files
         id: changed-files
         run: echo "changed_files=$(git diff --name-only -r HEAD^1 HEAD | xargs)" >> $GITHUB_OUTPUT
-      - uses: cachix/install-nix-action@7be5dee1421f63d07e71ce6e0a9f8a4b07c2a487 # v31
+      - uses: cachix/install-nix-action@a809471b5c7c913aa67bec8f459a11a0decc3fce # v31
         with:
           extra_nix_config: sandbox = true
           nix_path: nixpkgs=channel:nixpkgs-unstable

.github/workflows/pr.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
-    - uses: cachix/install-nix-action@7be5dee1421f63d07e71ce6e0a9f8a4b07c2a487 # v31
+    - uses: cachix/install-nix-action@a809471b5c7c913aa67bec8f459a11a0decc3fce # v31
       with:
         nix_path: nixpkgs=channel:nixos-unstable
         extra_nix_config: |

.github/workflows/update.yml

@@ -24,7 +24,7 @@ jobs:
         permissions: "contents:write"
         revoke_token: true
     - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
-    - uses: cachix/install-nix-action@7be5dee1421f63d07e71ce6e0a9f8a4b07c2a487 # v31
+    - uses: cachix/install-nix-action@a809471b5c7c913aa67bec8f459a11a0decc3fce # v31
       with:
         nix_path: nixpkgs=channel:nixos-unstable
         extra_nix_config: |