ci: bump cachix/install-nix-action from 20 to 21

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 20 to 21.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](https://github.com/cachix/install-nix-action/compare/v20...v21)

---
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/github_pages.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v3
-    - uses: cachix/install-nix-action@v20
+    - uses: cachix/install-nix-action@v21
       with:
         install_url: https://releases.nixos.org/nix/nix-2.13.3/install
         nix_path: nixpkgs=channel:nixos-unstable

.github/workflows/test.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v3
-    - uses: cachix/install-nix-action@v20
+    - uses: cachix/install-nix-action@v21
       with:
         install_url: https://releases.nixos.org/nix/nix-2.13.3/install
         nix_path: nixpkgs=channel:nixos-22.11

.github/workflows/update-flake.yml

@@ -12,7 +12,7 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v3
       - name: Install Nix
-        uses: cachix/install-nix-action@v20
+        uses: cachix/install-nix-action@v21
         with:
           install_url: https://releases.nixos.org/nix/nix-2.13.3/install
       - name: Update flake.lock