Attempt to prevent the .NET 6 build from picking up newer .NET versio…

…ns from the environment

This follows the recommended approach from https://github.com/actions/setup-dotnet

.github/workflows/build.yml

@@ -37,6 +37,8 @@ jobs:
       uses: actions/setup-dotnet@v3
       with:
         dotnet-version: ${{matrix.dotnet_version}}
+    - name: Create temporary global.json
+      run: echo '{"sdk":{"version": "${{ steps.stepid.outputs.dotnet-version }}"} }' > ./global.json
     - name: Install Apt Dependencies
       run: |
         sudo apt update