Replace instances of os.environ.get() where appropriate

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Current Behavior

We're using os.environ.get() in instances where the pipeline should arguably fail if the environment variable isn't set (e.g. creating the repo map).

Purpose

We should use os.environ[] in instances where there is no potential default value, and/or where the pipeline should fail if an environment variable isn't set.

Plan

• Look at all instances of os.environ.get and os.getenv and confirm that it's usage is correct
• Swap os.environ.get and os.getenv for os.environ[] where appropriate
• Decide between os.environ.get and os.getenv since os.getenv is just a wrapper to os.environ.get

Acceptance Criteria

• os.environ.get/os.getenv and os.environ are used appropriately in pipeline
• POPs team agrees on which jobs should/shouldn't fail if an environment variable isn't set in pipeline