add --upgrade to pip to get newer downloader (#4338)

.yamato/com.unity.ml-agents-performance.yml

   variables:
     UNITY_VERSION: {{ editor.version }}
   commands:
-    - python -m pip install unity-downloader-cli --index-url https://artifactory.prd.it.unity3d.com/artifactory/api/pypi/pypi/simple
+    - python -m pip install unity-downloader-cli --index-url https://artifactory.prd.it.unity3d.com/artifactory/api/pypi/pypi/simple --upgrade
     - unity-downloader-cli -u {{ editor.version }} -c editor --wait --fast
     - curl -s https://artifactory.internal.unity3d.com/core-automation/tools/utr-standalone/utr --output utr
     - chmod +x ./utr