[FLINK-38812][API/Python] Add log to show whether PYFLINK_CYTHON_ENABLED is set

[Myracle]

What is the purpose of the change

Add log to show whether PYFLINK_CYTHON_ENABLED is set

Brief change log

• Add log to show whether PYFLINK_CYTHON_ENABLED is set and show related info.

Verifying this change

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

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

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)

[flinkbot]

CI report:

• 1ffb1db Azure: SUCCESS

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[davidradl]

does this mean that if the environment variable is not specified we set it to True. If we want True to be the default, then would it not be clearer to set it to True prior to the if.

[davidradl]

Is it possible to add tests for the logic you have added please.