qremoteobjectnode.cpp: do not use auto when constructing QBA

It deduces the return type to QStringBuilder instead of QByteArray.

Originally pointed out by Axivion, which complains that it can cause
a crash. The complaint is not valid since
qtbase/af8f9a2a6e3dbc865470f8d7f09b533ccea4f353.

However, it is still better to explicitly use QByteArray here.
QStringBuilder does the concatenation every time the variable is used,
instead of doing it once. In this case the variable is used twice, so
using QStringBuilder is less efficient then immediately converting
the result to QByteArray.

Amends c03d630e6d9631791d0668b64025bd82796a55ce (picked to 6.9).

Axivion-Id: qt_Add-ons_qtremoteobjects_dev_linux:SV100
Fixes: QTBUG-144772
Pick-to: 6.12 6.11
Change-Id: I9f807bce7bfa921b02dc07f49d86e303eb38f9b6
Reviewed-by: Marc Mutz <marc.mutz@qt.io>