tmf: Use OSString for the supplementary files folder property

tmf: Use OSString for the supplementary files folder property

On Windows, getLocationURI().getPath() returns a string that
starts with "/C:/...". This does not get parsed correctly by
utilities like java.nio.file.Path.

Using getLocation().toOSString() instead solves this problem.

Change-Id: I7c5d6d43cd21fd9c0021105884f61fb32bd133d5
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/47932
Reviewed-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>