The userscript is a bash script and there is no bash on windows.
One solution could be to use a python userscript, but there may be
other issues (file associations), too.
On windows, using '/' in pathnames won't work, so it's impossible to use
to describe a path in a feature spec. The solution is to move the path
logic out of the feature spec and hand it over to `os.path.join` in a
new custom step for userscripts.
