Please remove the keras-nightly installation in the build containers

[seanpmorgan]

See this very cryptic error we get when using these containers given that installing tensorflow will give us a keras and keras-nightly installation in the containers: https://github.com/tensorflow/addons/pull/2635#issuecomment-1039647982

[angerson]

Pretty late on this -- I set up support for branches for the older releases. I'm not sure if that is going to fix this case, though. I'm expecting that the numbered containers (2.9-pythonX.Y...) are going to have keras-nightly until that release branch is cut, after which it's probably going to have keras= instead. That doesn't solve your issue, right?