Fix GitHub Actions CI

Microsoft apt repos break apt-get update for several days at every update, and there is no plan to fix these. 
As we are not using the repos, the best course of action is just to remove them. 

See: 
* actions/runner-images#323
* dotnet/core#4167

.github/workflows/ci.yml

@@ -25,6 +25,13 @@ jobs:
     - name: Environment Variables
       shell: bash
       run: env
+
+    # Remove apt repos that are known to break from time to time 
+    # See https://github.com/actions/virtual-environments/issues/323  
+    - name: Remove broken apt repos [Ubuntu]
+      if: matrix.os == 'ubuntu-latest'
+      run: |
+        for apt_file in `grep -lr microsoft /etc/apt/sources.list.d/`; do sudo rm $apt_file; done
         
     # ============
     # DEPENDENCIES