fix: lint migration github action failed for pull request from contributor

.github/workflows/lint-migrations.yml

@@ -10,9 +10,11 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v1
+      - name: Fetch main branch
+        run: git fetch origin main:main
       - name: Find modified migrations
         run: |
-          modified_migrations=$(git diff --diff-filter=d --name-only origin/$GITHUB_BASE_REF...origin/$GITHUB_HEAD_REF 'packages/db/migrations/*.do.*.sql')
+          modified_migrations=$(git diff --diff-filter=d --name-only main 'packages/db/migrations/*.do.*.sql')
           echo "$modified_migrations"
           echo "::set-output name=file_names::$modified_migrations"
         id: modified-migrations