CI: activate Github actions for pushes to backport branch (1.2.x)
xref https://github.com/pandas-dev/pandas/issues/39295#issuecomment-775329397

> for the Github actions issue, I should open a new issue and xref #38360. we may want to activate Github actions for pushes to 1.2.x and update `.github\workflows\ci.yml` which will currently upload docs on a push event...

```
    - name: Upload web
      run: rsync -az --delete --exclude='pandas-docs' --exclude='docs' --exclude='Pandas_Cheat_Sheet*' web/build/ docs@${{ secrets.server_ip }}:/usr/share/nginx/pandas
      if: github.event_name == 'push'
```
