Bump cachix/install-nix-action from 25 to 26

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 25 to 26.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](https://github.com/cachix/install-nix-action/compare/v25...v26)

---
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/ci.yaml

@@ -14,7 +14,7 @@ jobs:
             exit 1
           fi
       - name: Install nix
-        uses: cachix/install-nix-action@v25
+        uses: cachix/install-nix-action@v26
         with:
           nix_path: nixpkgs=channel:nixos-23.05
       - name: Check documentation site builds

.github/workflows/deploy-documentation.yml

@@ -30,7 +30,7 @@ jobs:
       - name: Setup Pages
         uses: actions/configure-pages@v4
       - name: Install nix
-        uses: cachix/install-nix-action@v25
+        uses: cachix/install-nix-action@v26
         with:
           nix_path: nixpkgs=channel:nixos-23.05
       - name: Build