export: Fix replacement of multiple TARGET_NAME expressions

After replacing each such expression, search for following expressions
immediately after the end of the replacement text, not after the
position of the end of the replaced text.

Fixes: #21661

Source/cmExportFileGenerator.cxx

@@ -707,7 +707,7 @@ void cmExportFileGenerator::ResolveTargetsInGeneratorExpression(
       break;
     }
     input.replace(pos, endPos - pos + 1, targetName);
-    lastPos = endPos;
+    lastPos = pos + targetName.size();
   }
 
   pos = 0;