chore(deps): 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](https://github.com/cachix/install-nix-action/compare/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] <support@github.com>

.github/workflows/check.yml

@@ -10,7 +10,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@v3
-      - uses: cachix/install-nix-action@v18
+      - uses: cachix/install-nix-action@v20
         with:
           install_url: https://nixos.org/nix/install
           extra_nix_config: |

.github/workflows/fmt.yml

@@ -10,7 +10,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@v3
-      - uses: cachix/install-nix-action@v18
+      - uses: cachix/install-nix-action@v20
         with:
           install_url: https://nixos.org/nix/install
           extra_nix_config: |