run.command(): Change to shebang handling

[Lestropie]

Revision to #2966 as described in comment.

If the command being run is an MRtrix3 executable (as established by the construction of that list at build time), and it is written in Python, then use the same Python interpreter as that currently executing. In any other circumstance, honour the shebang as specified in the command being invoked.

This is I think:

• Simpler
• Predictable
• Leaves execution power in hands of any third party tools being invoked
• Still won't cause weird Python version mismatch problems for any MRtrix3 executable code, whether invoked with the system default or any other interpreter.

Leaving as draft because I'll try to do some cross-checking on my Windows system first (and shouldn't be merged before #2859 revisions).

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"