[CI] Re-examine node/yarn install logic for CI

[brianseeders]

I split out setup_env, because I need to set up all of the environment stuff every time sh is called in the Jenkinsfile.

I was originally having issues with the parallel workers each trying to re-install node/yarn, so I introduced an argument to skip node/yarn installing, and just set up the env. But now that I look at the script again, I'm not sure why that was happening in the first place. Will take a second look

Originally posted by @brianseeders in #45285