Add env var for pip constraints

.github/workflows/deploy.yml

         os: [ubuntu-latest, macos-latest, windows-latest]
         python-version: ['3.6', '3.7', '3.8']
         pip-constraints: ['test_constraints_max_tf1_version.txt', 'test_constraints_max_tf2_version.txt', 'test_constraints_min_version.txt']
+    env:
+      PIP_CONSTRAINTS: ${{ matrix.pip-constraints }}
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python
     - name: Combine pip dependencies for caching
-      run: cat ml-agents/setup.py ml-agents-envs/setup.py gym-unity/setup.py test_requirements.txt ${{ matrix.pip_constraints }} > python_deps.txt
+      run: cat ml-agents/setup.py ml-agents-envs/setup.py gym-unity/setup.py test_requirements.txt $PIP_CONSTRAINTS > python_deps.txt
     - uses: actions/cache@v2
       if: startsWith(runner.os, 'Linux')
       with:
         . venv/bin/activate
         pip install --upgrade pip
         pip install --upgrade setuptools
-        pip install --progress-bar=off -e ./ml-agents-envs -c ${{ matrix.pip_constraints }}
-        pip install --progress-bar=off -e ./ml-agents -c ${{ matrix.pip_constraints }}
-        pip install --progress-bar=off -r test_requirements.txt -c ${{ matrix.pip_constraints }}
-        pip install --progress-bar=off -e ./gym-unity -c ${{ matrix.pip_constraints }}
+        pip install --progress-bar=off -e ./ml-agents-envs -c $PIP_CONSTRAINTS
+        pip install --progress-bar=off -e ./ml-agents -c $PIP_CONSTRAINTS
+        pip install --progress-bar=off -r test_requirements.txt -c $PIP_CONSTRAINTS
+        pip install --progress-bar=off -e ./gym-unity -c $PIP_CONSTRAINTS
     - name: Run Tests for ml-agents and gym_unity
       run: |
         . venv/bin/activate