Update actions/checkout action to v2 (#178)

Co-authored-by: Renovate Bot <[email protected]>

Author: renovate[bot]

.github/workflows/dart.yml

@@ -15,7 +15,7 @@ jobs:
         os: [ubuntu-latest]
 #        os: [ubuntu-latest, windows-latest, macos-latest]
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
       - uses: actions/setup-java@v1
         with:
           java-version: '12.x'