ci: upgrade to actions/checkout@v2

In particular, this appears to fix an extremely annoying bug that was
causing PR builds to fail if they were re-run.

For more details:
https://github.com/actions/checkout/issues/23#issuecomment-572688577

.github/workflows/ci.yml

@@ -87,7 +87,7 @@ jobs:
           rust: nightly-x86_64-gnu
     steps:
     - name: Checkout repository
-      uses: actions/checkout@v1
+      uses: actions/checkout@v2
 
     - name: Install packages (Ubuntu)
       if: matrix.os == 'ubuntu-18.04'
@@ -184,7 +184,7 @@ jobs:
     runs-on: ubuntu-18.04
     steps:
     - name: Checkout repository
-      uses: actions/checkout@v1
+      uses: actions/checkout@v2
     - name: Install Rust
       uses: actions-rs/toolchain@v1
       with:

.github/workflows/release.yml

@@ -108,7 +108,7 @@ jobs:
 
     steps:
     - name: Checkout repository
-      uses: actions/checkout@v1
+      uses: actions/checkout@v2
       with:
         fetch-depth: 1