Bump nixbuild/nix-quick-install-action from 19 to 20

Bumps [nixbuild/nix-quick-install-action](https://github.com/nixbuild/nix-quick-install-action) from 19 to 20.
- [Release notes](https://github.com/nixbuild/nix-quick-install-action/releases)
- [Changelog](https://github.com/nixbuild/nix-quick-install-action/blob/master/RELEASE)
- [Commits](https://github.com/nixbuild/nix-quick-install-action/compare/v19...v20)

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

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/build.yml

@@ -55,7 +55,7 @@ jobs:
         run: yarn
 
       - name: Install Nix
-        uses: nixbuild/nix-quick-install-action@v19
+        uses: nixbuild/nix-quick-install-action@v20
 
       - name: Setup Cachix
         if: github.event_name == 'push' && github.repository_owner == 'stephank'