Bump cachix/install-nix-action from 19 to 20 (#36)

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 19 to 20.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](https://github.com/cachix/install-nix-action/compare/v19...v20)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/nix.yml

@@ -14,6 +14,6 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v3
-      - uses: cachix/install-nix-action@v19
+      - uses: cachix/install-nix-action@v20
       - run: tests/run-tests.sh