Add a step to the asset copying task that makes a copy of
the guide-specific XSL file, replacing @p4-assets-dir@ with
the runtime property value. This facilitates building
documentation outside of the p4-doc depot hierarchy.
As the copied XSL lives in the same directory as the original,
there should be no effect for any existing XSL files. A future
changelist will update guide-specific XSL files to include the
property marker for consistency/flexibility.