VfsCopy: Up to date check fails due to wrong path

[ysb33r]

When checking paths to determine up to date status in multi-layer schemes, the wrong path is calculated, which results in the task always being out of date.

For instance the path build/download/revealjs-backends/reveal.js/section.html.slim is calculatedm whereas the path where the file was copied to is build/download/revealjs-backends/reveal.js/section.html.slim.