qmlimportscanner: Ensure the correct variant is run

qmlimportscanner is run as a $$system command (as opposed
to as a makefile command) so specify it as such to
qtPrepareTool and run it as such too.

The distinction is important for MinGW-w64 static
builds where it must be run via cmd.exe and not sh.exe.

Change-Id: I0832d5138bff7f4fa1968646df28d2367ad062c2
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>