find_program: Restore leading double slash on Windows network path
Since !2120 (merged) we accidentally convert the leading //
to just /
on results with Windows network paths.
This was caused by CollapseCombinedPath
being less robust than CollapseFullPath
. Restore use of the latter but in a way that preserves the requirement of the above commit to support candidates that are themselves absolute.
Fixes: #19049 (closed)
Issue: #19050 (closed)