fix: name 'pluginparent' is not defined

[capric98]

When using VapourSynth Portable with Python embeded version, is_sitepackage_install_portable() returns True, so https://github.com/vapoursynth/vsrepo/blob/1f89e14339122682b7164cd6ce8bc43e4cdb4607/vsrepo.py#L159-L175 will result in NameError: name 'pluginparent' is not defined in https://github.com/vapoursynth/vsrepo/blob/1f89e14339122682b7164cd6ce8bc43e4cdb4607/vsrepo.py#L177

If I use vsrepo {command} -p, the installed plugins will be saved to os.getcwd()/vapoursynth64/plugins or {vsrepo_path}/vapoursynth64/plugins depending on where vsrepo is installed.

Actually, if we define pluginparent, we could simply use vsrepo {command} without -p parameter, at lease the plugins will be installed to {python_path}/vapoursynth64/plugins.