[fix][broker] ensure new line added before appending new keys from env

[marksilcox]

Motivation

Copying change from apply-config-from-env.py to ensure new line included before appending

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): ( no)
• The public API: ( no)
• The schema: ( no)
• The default values of configurations: ( no)
• The wire protocol: ( no)
• The rest endpoints: ( no)
• The admin cli options: ( no)
• Anything that affects deployment: ( no )

Documentation

Need to update docs?

• no-need-doc
  (Please explain why)