Metadata queries broken in Windows

[rafatower]

Story details: https://app.clubhouse.io/cartoteam/story/69476

There are other ways of solving this. For a full discussion see https://stackoverflow.com/questions/1793261/how-to-join-components-of-a-path-when-you-are-constructing-a-url-in-python

but IMHO not worth the complexity, plus format is clear and kind of standard throughout the code.

[Jesus89]

Tests have failed, so we should review them before the release.