find_program: Restore leading double slash on Windows network path

Since commit c76c1ea208 (find_program: Consider CWD only for paths with
separator, 2018-05-31, v3.13.0-rc1~413^2) 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
Issue: #19050

Source/cmFindProgramCommand.cxx

@@ -77,7 +77,7 @@ struct cmFindProgramHelper
       this->TestNameExt = name;
       this->TestNameExt += ext;
       this->TestPath =
-        cmSystemTools::CollapseCombinedPath(path, this->TestNameExt);
+        cmSystemTools::CollapseFullPath(this->TestNameExt, path);
 
       if (cmSystemTools::FileExists(this->TestPath, true)) {
         this->BestPath = this->TestPath;