The execution model of jobs is that the user defines a "job wide" working directory via runtime_envs. However, this recent PR breaks this by overwriting the entire job runtime_env when the user specifies a runtime_env option.

This means that if you try to set the env_vars for a task/actor, you get "ModuleNotFoundError", with no obvious way to figure out why.

This was caused by #24538

We should:

1. Revert the PR pending further discussion, to unbreak master, and
2. Ensure we have adequate tests to prevent this regression in the future.