build(deps): bump cachix/install-nix-action from 19 to 20

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](cachix/install-nix-action@v19...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]>

.github/workflows/check-maintainers-sorted.yaml

@@ -16,7 +16,7 @@ jobs:
         with:
           # pull_request_target checks out the base branch by default
           ref: refs/pull/${{ github.event.pull_request.number }}/merge
-      - uses: cachix/install-nix-action@v19
+      - uses: cachix/install-nix-action@v20
         with:
           # explicitly enable sandbox
           extra_nix_config: sandbox = true