CI: Add debug version of ruby

Author: mrkn

.github/workflows/ci.yml

@@ -40,6 +40,8 @@ jobs:
         - { os: ubuntu-20.04   , ruby: 2.7   , python: 3.7 , python_architecture: x64 }
         - { os: ubuntu-20.04   , ruby: 2.7   , python: 3.6 , python_architecture: x64 }
         - { os: ubuntu-18.04   , ruby: 2.7   , python: 3.8 , python_architecture: x64 }
+        - { os: ubuntu-20.04   , ruby: debug , python: 3.x , python_architecture: x64 }
+        - { os: macos-latest   , ruby: debug , python: 3.x , python_architecture: x64 }
         - { os: windows-latest , ruby: mswin , python: 3.x , python_architecture: x64 }
         - { os: windows-latest , ruby: mingw , python: 3.x , python_architecture: x64 }