fix installer on standalone

.yamato/standalone-build-test.yml

   commands:
     - pip install pyyaml --index-url https://artifactory.prd.it.unity3d.com/artifactory/api/pypi/pypi/simple
     # TODO remove the "--user" command and the path prefix when we can migrate away from the custom bokken image
-    - python -m pip install unity-downloader-cli --index-url https://artifactory.prd.it.unity3d.com/artifactory/api/pypi/pypi/simple --upgrade --user
-    - /Users/bokken/Library/Python/3.7/bin/unity-downloader-cli -u {{ config.version }} -c editor --wait --fast
+    - 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 {{ config.version }} -c editor --wait --fast
     - python -u -m ml-agents.tests.yamato.standalone_build_tests
     - python -u -m ml-agents.tests.yamato.standalone_build_tests --scene=Assets/ML-Agents/Examples/Basic/Scenes/Basic.unity
     - python -u -m ml-agents.tests.yamato.standalone_build_tests --scene=Assets/ML-Agents/Examples/Bouncer/Scenes/Bouncer.unity

.yamato/test_versions.metafile

 # List of configurations for standalone-build-test and its dependencies.
 # csharp_backcompat_version is used in training-int-tests to determine the
 # older package version to run the backwards compat tests against.
+# TODO either separate dictionary for platforms (type, image, flavor), or list the versions to test by platform.
 test_configs:
   - name: Mac_2018.4
     version: 2018.4
  #   # but we didn't handle this until 1.2.0
  #   csharp_backcompat_version: 1.2.0
 
+ # TODO skip some tests on Windows based on their triggers.
   - name: Win_2019.4
     version: 2019.4
     type: Unity::VM