Add Forest installation in dev mode to GitHub Actions build workflow

.github/workflows/build.yml

@@ -14,6 +14,8 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: 3.6
+      - name: Install Forest
+        run: pip install -e .
       - name: Install dev dependecies
         run: pip install -r requirements.txt
       - name: Run code style checking