Update workflows to actions/checkout@v3

Author: dtolnay

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
       matrix:
         rust: [nightly, beta, stable]
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@master
         with:
           toolchain: ${{matrix.rust}}