try self-hosted CI for dotnet on mac OS

.github/scripts/mac-action-runner-readme.txt

@@ -21,3 +21,6 @@ JDK (we need a specific JDK):
  - fix the JAVA_HOME mess as follows:
     sudo rm -fr /Library/Internet\ Plug-Ins/JavaAppletPlugin.plugin
     sudo rm -fr /Library/PreferencePanes/JavaControlPanel.prefpane
+
+C++:
+ - brew install cmake

.github/workflows/macosx.yml

@@ -15,7 +15,7 @@ jobs:
  #       GROUP: [LEXER, PARSER, RECURSION]
  #       TARGET: [swift, cpp, dotnet]
         GROUP: [LEXER]
-        TARGET: [cpp]
+        TARGET: [dotnet]
     steps:
     - uses: actions/checkout@v2
     - name: Set up JDK 1.8