move everything except training tests to linux

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

 pack:
   name: Pack
   agent:
-    type: Unity::VM::osx
-    image: package-ci/mac:stable
+    type: Unity::VM
+    image: package-ci/ubuntu:stable
     flavor: b1.small
   commands:
     - npm install upm-ci-utils@stable -g --registry https://artifactory.prd.cds.internal.unity3d.com/artifactory/api/npm/upm-npm

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

     - .yamato/com.unity.ml-agents-pack.yml#pack
   triggers:
     cancel_old_ci: true
-    {% if platform.name == "mac" %}
+    {% if platform.name == "linux" %}
     expression: |
       (pull_request.target eq "master" OR
       pull_request.target match "release.+") AND

.yamato/compressed-sensor-test.yml

 test_compressed_obs_{{ editor.version }}:
   name: Test Compressed Sensor Observation {{ editor.version }}
   agent:
-    type: Unity::VM::osx
-    image: ml-agents/ml-agents-bokken-mac:0.1.4-492264
+    type: Unity::VM
+    image: package-ci/ubuntu:stable
     flavor: b1.small
   variables:
     UNITY_VERSION: {{ editor.version }}

.yamato/gym-interface-test.yml

 test_gym_interface_{{ editor.version }}:
   name: Test Mac Gym Interface {{ editor.version }}
   agent:
-    type: Unity::VM::osx
-    image: ml-agents/ml-agents-bokken-mac:0.1.4-492264
+    type: Unity::VM
+    image: package-ci/ubuntu:stable
     flavor: b1.small
   variables:
     UNITY_VERSION: {{ editor.version }}

.yamato/python-ll-api-test.yml

 test_mac_ll_api_{{ editor.version }}:
   name: Test Mac LL-API {{ editor.version }}
   agent:
-    type: Unity::VM::osx
-    image: ml-agents/ml-agents-bokken-mac:0.1.4-492264
+    type: Unity::VM
+    image: package-ci/ubuntu:stable
     flavor: b1.small
   variables:
     UNITY_VERSION: {{ editor.version }}

.yamato/standalone-build-test.yml

 test_mac_standalone_{{ editor.version }}:
   name: Test Mac Standalone {{ editor.version }}
   agent:
-    type: Unity::VM::osx
-    image: ml-agents/ml-agents-bokken-mac:0.1.4-492264
+    type: Unity::VM
+    image: package-ci/ubuntu:stable
     flavor: i1.small
   variables:
     UNITY_VERSION: {{ editor.version }}

ml-agents/tests/yamato/yamato_utils.py

 import shutil
 import subprocess
 import yaml
+from sys import platform
 from typing import List, Optional
 
 
     pip_commands = ["--upgrade pip", "--upgrade setuptools"]
     if mlagents_python_version:
         # install from pypi
+        if platform != "darwin":
+            raise RuntimeError("Yamato can only run tensorflow on mac platforms!")
         pip_commands += [
             f"mlagents=={mlagents_python_version}",
             f"gym-unity=={mlagents_python_version}",