Bump cachix/install-nix-action from 18 to 20

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 18 to 20.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](cachix/install-nix-action@v18...v20)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/nix.yml

@@ -50,7 +50,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: cachix/install-nix-action@v18
+      - uses: cachix/install-nix-action@v20
         with:
           nix_path: ${{ env.NIX_CHANNEL }}
           install_url: ${{ env.NIX_INSTALL_URL }}
@@ -83,7 +83,7 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - name: Install Nix ❄️
-        uses: cachix/install-nix-action@v18
+        uses: cachix/install-nix-action@v20
         with:
           extra_nix_config: "system-features = nixos-test benchmark big-parallel kvm"
           nix_path: ${{ env.NIX_CHANNEL }}