Build: show "staff" UI elements

[humitos]

It would be good to add those UI elements that are only show to staff users. The one that I'm needing now is the build.builder, but there may be others: https://github.com/readthedocs/readthedocs.org/blob/7a58acb50b964bc60f19ea10a1e5135737d77d84/readthedocs/templates/builds/build_detail.html#L57-L62