Bump actions/cache from v1 to v2

.github/workflows/main.yml

@@ -50,7 +50,7 @@ jobs:
         run: poetry config virtualenvs.in-project true
 
       - name: Set up cache
-        uses: actions/cache@v1
+        uses: actions/cache@v2
         id: cache
         with:
           path: .venv