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/build_nix.yml

@@ -17,7 +17,7 @@ jobs:
           access_token: ${{ github.token }}
 
       - name: Install Nix
-        uses: cachix/install-nix-action@v18
+        uses: cachix/install-nix-action@v20
 
       - name: Checkout Repository
         uses: actions/checkout@v3