Misc: Make the conversion script executable

[Dietr1ch]

Not much, but has some benefits,

• Shorter commands.
• Help actual executable files to stand out.

[Dietr1ch]

You can ask for python3 here if needed, but I feel it's not necessary since the Python 2 sunset was on 2020,

https://www.python.org/doc/sunset-python-2/

[ggerganov]

I prefer the python commands to be explicit

[Dietr1ch]

You can keep using python convert-pth-to-ggml.py if you want, the shebang is just a regular python comment.