ci: brew install libiconv for test-examples on macos

This started failing suddenly, I imagine because Github's base
container removed libiconv or something.

.github/workflows/ci.yml

@@ -71,6 +71,7 @@ jobs:
         with:
           apt-get: _update_ build-essential cmake
           mingw: _upgrade_ cmake
+          brew: libiconv
           ruby-version: ${{ matrix.ruby }}
           bundler-cache: true
       - uses: actions/cache@v4