Upgrade actions/checkout to v3

Author: JohnTitor

.github/workflows/main.yml

@@ -6,7 +6,7 @@ jobs:
     name: Test
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Update rustup
       run: rustup self update
     - name: Install Rust