upgrade checkout action version

Author: softprops

.github/workflows/main.yml

@@ -96,7 +96,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
       - name: Checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
       - name: Setup Rust
         uses: hecrj/setup-rust-action@v1
         with: