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

Author: renovate[bot]

.github/workflows/gh-pages.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
-      - uses: cachix/install-nix-action@v20
+      - uses: cachix/install-nix-action@v31
       - uses: cachix/cachix-action@v12
         with:
           name: numtide