Fix bug in source verification on Windows

The way to pass the set of directories to be used for validating the source caused issues on Windows systems, as it replaced ":" characters to change from strings to lists in CMake.

This caused the top level directory to be changed from e.g. "D:" to "D;", messing up the rest.

Fixed the message passing to only pass the final directory, and modify to full path after the set has been passed through.

Also fixes path for regressiontest download.

Fixes #3493 (closed)