Add checkout step to GitHub Actions workflow

The checkout step is now included in the workflow to ensure the repository is available for subsequent steps. This change is crucial for successful artifact uploads and overall workflow execution.

.github/workflows/main.yml

@@ -64,6 +64,9 @@ jobs:
       matrix:
         artifact_name: [ "HI2UC", "HIColorer" ]
     steps:
+      - name: Checkout repository
+        uses: actions/checkout@v3
+
       - name: Upload ${{matrix.artifact_name}}
         uses: ./.github/workflows/plugin-upload
         with: