This pull request improves the internal code by removing os.environment access in default arguments. Although I removed most of such code in early development, it seems I failed to completely remove them at that time.

Default parameter values are evaluated from left to right when the function definition is executed. This means that the expression is evaluated once, when the function is defined, and that the same “pre-computed” value is used for each call.

https://docs.python.org/3.8/reference/compound_stmts.html#function-definitions

Category (place an x in each of the [ ])

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter

Requirements (place an x in each [ ])

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• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run ./scripts/run_tests.sh after making the changes.