fix: force to use / as a path separator in yarn-path

[exKAZUu]

Summary

Close #7777 Close yarnpkg/berry#858

\\ in yarn-path causes a problem where berry cannot be run on a non-Windows environment.
This PR forces to use / as a path separator in yarn-path even on Windows.

Test plan

Welcome to Node.js v13.3.0.
Type ".help" for more information.
> 'foo\\bar\\baz'.split(path.sep).join('/')
'foo/bar/baz'
> '\\foo\\bar\\baz'.split(path.sep).join('/')
'/foo/bar/baz'
> '\\foo\\bar\\baz\\'.split(path.sep).join('/')
'/foo/bar/baz/'
> '..\\foo\\bar\\baz\\'.split(path.sep).join('/')
'../foo/bar/baz/'

[arcanis]

Thanks! Fixed in #7862