ci: update actions/checkout

Author: orestesgaolin

.github/workflows/build.yaml

@@ -25,7 +25,7 @@ jobs:
             os: macos-13
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: dart-lang/setup-dart@v1
       - run: dart pub get
       - run: mkdir build