[rb-general] Clarification on BUILD_PATH_PREFIX_MAP and drive letters

[Dan Kegel]

https://reproducible-builds.org/specs/build-path-prefix-map/ says
  "The : character separates encoded list items from each other."
This seems to have forgotten about Windows, which uses : for drive
letters, and typically uses ; for such separators.
Presumably on Windows, a Ruby implementation would use
File::PATH_SEPARATOR for this character, and the spec needs a tweak.