Fix name 'pluginparent' is not defined in virtualenv Python

[machinewu]

Reproduce the issue in Win10:

1. Create a Python virtualenv (D:\virtual_py3)
2. Run the activate script(D:\virtual_py3\Scripts\activate.bat) to activate virtualenv. Then run pip install vapoursynth-portable to install vapoursynth.
3. Run python vsrepo.py update
4. Run python vsrepo.py install lsmas dfttest. Then an error occurred:

   Traceback (most recent call last):
   File "D:\virtual_py3\vsrepo\vsrepo.py", line 176, in <module>
   package_json_path = os.path.join(file_dirname, 'vspackages3.json') if args.portable else os.path.join(*pluginparent, 'vsrepo', 'vspackages3.json')
                                                                                                          ^^^^^^^^^^^^
   NameError: name 'pluginparent' is not defined

   We can find the file portable.vs in the site-packages dir: D:\virtual_py3\Lib\site-packages\portable.vs

if args.portable:
    plugin32_path = os.path.join(file_dirname, 'vapoursynth32', 'plugins')
    plugin64_path = os.path.join(file_dirname, 'vapoursynth64', 'plugins')
elif is_sitepackage_install_portable():
    vapoursynth_path = detect_vapoursynth_installation()
    base_path = os.path.dirname(vapoursynth_path)
    plugin32_path = os.path.join(base_path, 'vapoursynth32', 'plugins')
    plugin64_path = os.path.join(base_path, 'vapoursynth64', 'plugins')
    del vapoursynth_path
else:
    pluginparent = [str(os.getenv("APPDATA")), 'VapourSynth']
    plugin32_path = os.path.join(*pluginparent, 'plugins32')
    plugin64_path = os.path.join(*pluginparent, 'plugins64')

According to the logic of the code, the statement elif is_sitepackage_install_portable(): will be entered. But the variable pluginparent is not set here. So, the following code will throw an error NameError: name 'pluginparent' is not defined.

package_json_path = os.path.join(file_dirname, 'vspackages3.json') if args.portable or is_sitepackage_install_portable() else os.path.join(*pluginparent, 'vsrepo', 'vspackages3.json')