Specify extra library path to pyinstaller (fixes #17)

On Windows, pyinstaller was not seeing my globally installed site
packages, which included the 'yaml' module. By explicitly listing the
path, it corrects the issue with pyinstaller.

This was never version specific, but likely due to how python was
installed on my Windows environment.

build-win.bat

@@ -1,3 +1,3 @@
-pip install pyinstaller==3.6
-pyinstaller download.py -n gog-plugins-downloader --onefile
+pip install pyinstaller==4.5.1
+pyinstaller.exe download.py -n gog-plugins-downloader --onefile -p C:\Users\username\scoop\apps\python\current\Lib\site-packages\
 COPY gog-plugins.bat dist\