fix duplicate env vars + ability to add custom address for scheduler

[s-ducks]

fixes:

• Env var duplication #841
• Ability to add different scheduler address to workers outside of standard format #842 which duplicates:
• Reset Dask worker to use TCP even if it was configured to use TLS in yaml file #836
  alternative to:
• Prepend user env vars #837

why this approach?
I think it would be good to "prefer" the user set env vars (or env vars set from elsewhere), which is what the addition of _consolidate_env_vars does

[jacobtomlinson]

I think this has been superseded. If I'm wrong please comment and I can reopen.