Install nmigen dep in Github Action

.github/workflows/main.yml

@@ -8,6 +8,9 @@ jobs:
         uses: actions/checkout@v2
       - name: Set up Python
         uses: actions/setup-python@v2
+      - name: Install nmigen
+        run: |
+          pip install git+https://github.com/nmigen/nmigen
       - name: Test
         run: |
           python setup.py test