speedo,w32: Use nsExec::ExecToLog to avoid popups

* build-aux/speedo/w32/inst.nsi: Use ExecToLog instead of
ExecWait.

nsExec is a standard nsis call to spawn a process and
wait for it to finish. ExecToLog redirects stdout and stderr
of the process call and adds it to the log window instead
of opening a temporary console window.

• Signed-off-by: Andre Heinecke <aheinecke@intevation.de>