ci: isolate surge preview token

Author: afc163

.github/workflows/surge-preview.yml

@@ -24,10 +24,15 @@ jobs:
       - name: Install dependencies
         run: npm install --ignore-scripts --no-audit --loglevel=warn
 
+      - name: Build preview
+        if: ${{ secrets.SURGE_TOKEN != '' }}
+        run: npm run build
       - uses: afc163/surge-preview@bf90a5a86111f6311ca42f0a5a0f80fb0fb03cec
+        if: ${{ secrets.SURGE_TOKEN != '' }}
+        env:
+          SURGE_TOKEN: ${{ secrets.SURGE_TOKEN }}
         with:
-          build: npm run build
           dist: docs-dist
           failOnError: false
           github_token: ${{ secrets.GITHUB_TOKEN }}
-          surge_token: ${{ secrets.SURGE_TOKEN }}
+          surge_token: ${{ env.SURGE_TOKEN }}