Improve path resolution for extensions.

[psilentp]

The inkscape extensions appear to need to extend the system path to include the extension directory. We should come up with a cleaner way of doing this than hard coding guesses into the scripts. It is also worth determining if this is still needed.

https://github.com/FlyRanch/figurefirst/blob/c8cad3d18f2eaa62ed418649918bc3c0fbdaf252/inkscape_extensions/1.x/tag_axis.py#L3-L5