Do not evaluate qbs.architectures more often than necessary
Do not evaluate qbs.architectures more often than necessary

The rhs of this property is non-trivial. Try to minimize accesses to it.

Change-Id: I8b6fcfbc9feff501a26857ac12f5291d211b57ba
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@qt.io>

qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs