.github: bump install-nix-action (#3723)

The new release addresses the issue with Nix 2.14.

https://github.com/cachix/install-nix-action/releases/tag/v20

.github/workflows/github_pages.yml

@@ -11,9 +11,8 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v3
-    - uses: cachix/install-nix-action@v19
+    - uses: cachix/install-nix-action@v20
       with:
-        install_url: https://releases.nixos.org/nix/nix-2.13.3/install
         nix_path: nixpkgs=channel:nixos-unstable
     - uses: cachix/cachix-action@v12
       with:

.github/workflows/test.yml

@@ -12,9 +12,8 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v3
-    - uses: cachix/install-nix-action@v19
+    - uses: cachix/install-nix-action@v20
       with:
-        install_url: https://releases.nixos.org/nix/nix-2.13.3/install
         nix_path: nixpkgs=channel:nixos-unstable
     - uses: cachix/cachix-action@v12
       with:

.github/workflows/update-flake.yml

@@ -12,9 +12,7 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v3
       - name: Install Nix
-        uses: cachix/install-nix-action@v19
-        with:
-          install_url: https://releases.nixos.org/nix/nix-2.13.3/install
+        uses: cachix/install-nix-action@v20
       - name: Update flake.lock
         uses: DeterminateSystems/update-flake-lock@v17
         with: