PYTHON not set correctly in export.sh

[mikee47]

The PYTHON environment variable isn't set correctly for CI builds, or for other installations where non-standard locations may be used. This can result in unexpected behaviour where multiple python3 versions are installed.

Update to #2735.

[what-the-diff]

PR Summary

• Modification of 'export.sh' File
  The 'export.sh' file located in the 'Tools' directory has been updated. This change is integral to ensure the correct operation of our software tools.

• Environment Variable Update for PYTHON
  We've adjusted how the PYTHON environment variable is set. Now, it uses an automatic system command (which python3) to identify the location of Python3 on your computer. This enhancement makes it easier for the system to interface with Python, irrespective of where it's installed on your computer.