PCH: Fix regression in REUSE_FROM that copies the .pdb too often

Since commit 99bdf2ae16 (cmLocalGenerator: add PDB copying dependency on
its script, 2025-06-16, v4.2.0-rc1~481^2~8) the copy is repeated
whenever the copying script is updated.  Avoid updating the copy script
timestamp when it does not change.

Fixes: #27608

Source/cmLocalGenerator.cxx

@@ -2914,6 +2914,7 @@ void cmLocalGenerator::CopyPchCompilePdb(
   std::string const copy_script = cmStrCat(target->GetSupportDirectory(),
                                            "/copy_idb_pdb_", config, ".cmake");
   cmGeneratedFileStream file(copy_script);
+  file.SetCopyIfDifferent(true);
 
   file << "# CMake generated file\n";