chore: Update GitHub "checkout" action to v2

Attempting to fix the "reference is not a tree" error that has started occurring.

.github/workflows/build_and_test.yml

@@ -16,7 +16,7 @@ jobs:
     name: build
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
     - name: Use Node.js ${{ env.node }}
       uses: actions/setup-node@v1
       with: