Bump actions/setup-python to v2

Keep it up-to-date even though we are not currently
using features added in v2.

.github/workflows/build.yml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - uses: actions/setup-python@v1
+      - uses: actions/setup-python@v2
         with:
           python-version: 3.8
       - name: Store installed Python version