always create qt_tool_syncqt.pri when shadow-building

the fallback is to look in the install dir (which is the build dir when
no -prefix is used), which doesn't work for a script in the source dir.

amends 6b38524b



Task-number: QTBUG-31131
Change-Id: If615ab4e3a2839b6b5be9bf6ef79d3f894b1748d
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>