CI: Fix documentation generation for macOS

- Force Doxygen version 1.9.6, because 1.9.7 breaks our doc cool URIs,
  i.e. our system that maintain stable doc pages via redirection.

  Associated issue in Doxygen:
  doxygen/doxygen#10146

- Install graphviz to build the .dot files.

.github/workflows/macos.yml

@@ -24,7 +24,11 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade meson PyYAML
-          brew install libxml2 doxygen bison ninja
+          brew install libxml2 bison ninja graphviz
+          # HACK: force Doxygen version 1.9.6, because 1.9.7 breaks our cool URIs.
+          curl -O https://raw.githubusercontent.com/Homebrew/homebrew-core/d2267b9f2ad247bc9c8273eb755b39566a474a70/Formula/doxygen.rb
+          brew install --formula doxygen.rb
+          rm doxygen.rb
           brew link bison --force
         env:
           HOMEBREW_NO_AUTO_UPDATE: 1