Bump cachix/install-nix-action from 30 to 31

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 30 to 31.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](https://github.com/cachix/install-nix-action/compare/v30...v31)

---
updated-dependencies:
- dependency-name: cachix/install-nix-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/demo.yml

@@ -50,7 +50,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Install nix
-        uses: cachix/install-nix-action@v30
+        uses: cachix/install-nix-action@v31
 
       - uses: cachix/cachix-action@v15
         with:

.github/workflows/pages.yml

@@ -32,7 +32,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Install nix
-        uses: cachix/install-nix-action@v30
+        uses: cachix/install-nix-action@v31
 
       - name: Setup Caching
         uses: cachix/cachix-action@v15