CI: Nightly build is not running build_docs

[mattip]

PR gh-1006 added build_docs and upload_docs jobs, the former is meant to run on every PR, merge, and nightly run, and the latter on nightly and tags. But the build_docs does not run on nightly since it requires binary_linux_wheel_py3.8 and not one of the nightly jobs. One alternative would be to have build_docs_nightly and upload_docs_nightly that duplicate the current ones but use requires: nightly_binary_linux_wheel_py3.8

[mthrok]

Closing as this is addressed in #1033 and #1034.