Use `actions/checkout@v2`

Note that the repository's default branch is now `main`, not `master`.
And this should be safer to avoid breaking change.

.github/workflows/main.yml

@@ -6,7 +6,7 @@ jobs:
     name: Test
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v2
     - name: Update rustup
       run: rustup self update
     - name: Install Rust