updating explorer_preview.yml

it deploy preview on vercel

.github/workflows/explorer_preview.yml

@@ -10,6 +10,18 @@ jobs:
     if: ${{ github.event.workflow_run.conclusion == 'success' }}
     steps:
        - uses: actions/checkout@v2
+         with:
+          ref: ${{ github.event.pull_request.head.sha }}
+       -  uses: actions/setup-node@v2
+          with:
+           node-version: '14'
+           cache: 'npm'
+           cache-dependency-path: explorer/package-lock.json
+       - run: npm i -g npm@7
+       - run: npm ci
+       - run: npm run format
+       - run: npm run build
+       - run: npm run test
        - uses: amondnet/vercel-action@v20
          with:
           vercel-token: ${{ secrets.VERCEL_TOKEN }} # Required