chore(deps): update cachix/install-nix-action action to v31.7.0

Author: renovate[bot]

.github/workflows/CI.yml

@@ -16,7 +16,7 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
-      - uses: cachix/install-nix-action@a809471b5c7c913aa67bec8f459a11a0decc3fce # v31.6.2
+      - uses: cachix/install-nix-action@9280e7aca88deada44c930f1e2c78e21c3ae3edd # v31.7.0
       - name: Build
         run: nix-shell --pure --run 'make SPHINXOPTS='-W' html'
   build_docker_image_theme_tuleap_org: