feat: support multiple preview jobs

[afc163]

这样可以定义多个部署预览任务，name 随意自取。

可以解决：umijs/dumi#287 (comment)

  preview:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - uses: ./
        name: test afc163/surge-preview
        with:
          surge_token: ${{ secrets.SURGE_TOKEN }}
          github_token: ${{ secrets.GITHUB_TOKEN }}
          build: |
            npm install
            npm run build
          dist: public
  preview2:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - uses: ./
        name: test afc163/surge-preview
        with:
          surge_token: ${{ secrets.SURGE_TOKEN }}
          github_token: ${{ secrets.GITHUB_TOKEN }}
          build: |
            npm install
            npm run build
          dist: public

[github-actions]

🎊 PR Preview be3e3c7 has been successfully built and deployed to https://afc163-surge-preview-preview2-pr-16.surge.sh

🕐 Build time: 32.445s

_{🤖 By surge-preview}

[github-actions]

🎊 PR Preview be3e3c7 has been successfully built and deployed to https://afc163-surge-preview-preview-pr-16.surge.sh

🕐 Build time: 40.908s

_{🤖 By surge-preview}