Fix Xvfb Initialization

Author: maryamziaa

.github/workflows/colab.yml

@@ -53,6 +53,11 @@ jobs:
         python -m pip install --progress-bar=off -e ./ml-agents-envs
         python -m pip install --progress-bar=off -e ./ml-agents
         python -m pip install --progress-bar=off -r colab_requirements.txt
+    -   name: Start Xvfb
+        run: |
+            sudo apt-get install xvfb
+            Xvfb :99 -screen 0 1024x768x16 &
+            export DISPLAY=:99
     - name: Execute notebook
       run: jupyter nbconvert --to notebook --execute --log-level=DEBUG --ExecutePreprocessor.kernel_name=python3 --output output-${{ matrix.notebook_path }} colab/${{ matrix.notebook_path }}
     - name: Upload colab results