QFileSystemEngine: fix potential formatting error

It's not immediately clear that trashedName doesn't contain %1 or %2,
in which case the subsequent .arg() would replace that instead of the
intended %2.

Fix by using QString::asprintf(), which doesn't perform multiple
interpolation passes.

Pick-to: 6.3 6.2 5.15
Change-Id: Ib6c24bfea01db4cdc80f7547c8269cce3f815158
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>

src/corelib/io/qfilesystemengine_unix.cpp

@@ -1374,10 +1374,7 @@ bool QFileSystemEngine::moveFileToTrash(const QFileSystemEntry &source,
     int counter = 0;
     QFile infoFile;
     auto makeUniqueTrashedName = [trashedName, &counter]() -> QString {
-        ++counter;
-        return QString(QLatin1String("/%1-%2"))
-                                        .arg(trashedName)
-                                        .arg(counter, 4, 10, QLatin1Char('0'));
+        return QString::asprintf("/%ls-%04d", qUtf16Printable(trashedName), ++counter);
     };
     do {
         while (QFile::exists(trashDir.filePath(filesDir) + uniqueTrashedName))