When running without installing, extensions will spawn correct Inkscape

As noted by @su-v , when Inkscape is started from a custom location, and extensions that open new Inkscape instances are run, they will need to spawn the correct Inkscape, else one can get errors due to incompatible inkex.py files. The fix is to prepend the currently used Inkscape version to the PATH variable.

(see also: https://bugs.launchpad.net/inkscape/+bug/1525418/comments/21)