Ensure use of python from venv on Mac and Linux

[viking1304]

Description

If the user sets the full path to the Python executable in webui-user.sh, venv will be created as usual. Everything will work and look normal, but all packages will be actually installed globally.

It fixes #16113

The reason for this behavior is that in this line

"${python_cmd}" -u "${LAUNCH_SCRIPT}" "$@"

python_cmd contains the full path and will be executed as

/opt/homebrew/bin/python3.10 -u launch. py

instead of

python3.10 -u launch. py

This change ensures that the python executable from venv will be used, even if the user sets the full path. This change only affects users who use venv; it does not change anything if venv is not used.

Screenshots/videos:

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests