fix names, fix python3 commands, build player on mac backcompat

.yamato/gym-interface-test.yml

 ---
 {% for editor in test_editors %}
 test_gym_interface_{{ editor.version }}:
-  name: Test Mac Gym Interface {{ editor.version }}
+  name: Test Linux Gym Interface {{ editor.version }}
   agent:
     type: Unity::VM
     image: package-ci/ubuntu:stable

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

 ---
 {% for editor in test_editors %}
 test_mac_ll_api_{{ editor.version }}:
-  name: Test Mac LL-API {{ editor.version }}
+  name: Test Linux LL-API {{ editor.version }}
   agent:
     type: Unity::VM
     image: package-ci/ubuntu:stable

.yamato/training-int-tests.yml

     # Backwards-compatibility tests.
     # If we make a breaking change to the communication protocol, these will need
     # to be disabled until the next release.
+    - python3 -u -m ml-agents.tests.yamato.standalone_build_tests --build-target=mac
     - python -u -m ml-agents.tests.yamato.training_int_tests --python=0.16.0
     - python -u -m ml-agents.tests.yamato.training_int_tests --csharp={{ csharp_backcompat_version }}
   dependencies:

ml-agents/tests/yamato/yamato_utils.py

         venv_path += "_" + mlagents_python_version
 
     # Set up the venv and install mlagents
-    subprocess.check_call(f"python -m venv {venv_path}", shell=True)
+    subprocess.check_call(f"python3 -m venv {venv_path}", shell=True)
     pip_commands = ["--upgrade pip", "--upgrade setuptools"]
     if mlagents_python_version:
         # install from pypi
     for cmd in pip_commands:
         pip_index_url = "--index-url https://artifactory.prd.it.unity3d.com/artifactory/api/pypi/pypi/simple"
         subprocess.check_call(
-            f"source {venv_path}/bin/activate; python -m pip install -q {cmd} {pip_index_url}",
+            f"source {venv_path}/bin/activate; python3 -m pip install -q {cmd} {pip_index_url}",
             shell=True,
         )
     return venv_path