Use shutil.which instead of calling out to subprocess

This MR patches pesummary.core.webpage.main._WebpageGeneration.get_executable() to use shutil.which() instead of calling out to subprocess and also appends the parent directory of the current python executable to the search path. This should mean that we can resolve executable paths even when PATH has no contents at all, which is useful when calling pesummary by just using the full path of an executable (and not activateing an environment.