Remove some files on GitHub Action runner to free up some space

Our CI jobs constantly fail due to the lack of disk space, so free up
some space by removing unnecessary directories following suggestions at
https://github.com/actions/runner-images/issues/2840

.github/workflows/ci.yml

@@ -252,6 +252,9 @@ jobs:
 
           echo "wxMAKEFILE_CXXFLAGS=$wxMAKEFILE_CXXFLAGS $error_opts" >> $GITHUB_ENV
 
+          # Remove some unnecessary files to ensure we have enough free space.
+          sudo rm -rf "AGENT_TOOLSDIRECTORY" /usr/share/dotnet
+
           ./build/tools/before_install.sh
 
       - name: Show build environment
@@ -268,6 +271,10 @@ jobs:
           ccache --version
           echo
 
+          echo "Free space:"
+          df -h /
+          echo
+
       - name: Configuring
         run: |
           wxCONFIGURE_OPTIONS="--disable-optimise $wxCONFIGURE_FLAGS"