GitHub: free more disk space

Workflow "Build" has started failing with out of disk space errors.
Expand the existing efforts to free up space (removal of some .NET,
Android, and GHC files).  Use advice from

    actions/runner-images#709 (comment)

and remove also C++ Boost library files and some GitHub's own Python
tools.

.github/workflows/build.yml

@@ -39,6 +39,8 @@ jobs:
           sudo rm -rf /usr/share/dotnet
           sudo rm -rf /usr/local/lib/android
           sudo rm -rf /opt/ghc
+          sudo rm -rf /usr/local/share/boost
+          sudo rm -rf "$AGENT_TOOLSDIRECTORY"
 
       # Check out current repository
       - name: Fetch Sources