ci: upgrade to actions/checkout@v2

For more details:
actions/checkout#23 (comment)

Author: BurntSushi

.github/workflows/ci.yml

@@ -44,7 +44,7 @@ jobs:
           rust: stable
     steps:
     - name: Checkout repository
-      uses: actions/checkout@v1
+      uses: actions/checkout@v2
       with:
         fetch-depth: 1
     - name: Install Rust
@@ -60,7 +60,7 @@ jobs:
     runs-on: ubuntu-18.04
     steps:
     - name: Checkout repository
-      uses: actions/checkout@v1
+      uses: actions/checkout@v2
       with:
         fetch-depth: 1
     - name: Install Rust