Use actions/checkout v2 for Rustfmt

Allows changes to behavior of this action to be more visible as dependabot updates it

Author: spenserblack

Diff for: .github/workflows/ci.yml

@@ -38,7 +38,7 @@ jobs:
     name: Rustfmt
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v2
     - name: Install Rust
       uses: actions-rs/toolchain@v1
       with: