Bump actions/checkout from 2 to 4 (#35)

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/build.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Set up Git repository
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
       - name: prepare papers
         working-directory: ./papers
         run: |