[CK Tools] Auto-enable unbuffered output for Python commands
(#4265)
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ck-docker exec and ck-exec now automatically detect Python commands and
set PYTHONUNBUFFERED=1 to enable live output streaming. This eliminates
the need to manually set the environment variable when running Python
scripts that print progress updates.
The detection matches python, python3, or any .py file argument.
This helps in watching live terminal output when a python script is
running inside the container.
