Fix raster calc dialog uses an open file dialog instead of save dialog

Fixes #18303

Author: nyalldawson

src/app/qgsrastercalcdialog.cpp

@@ -77,6 +77,7 @@ QgsRasterCalcDialog::QgsRasterCalcDialog( QgsRasterLayer *rasterLayer, QWidget *
   mExpressionTextEdit->setCurrentFont( QFontDatabase::systemFont( QFontDatabase::FixedFont ) );
 
   QgsSettings settings;
+  mOutputLayer->setStorageMode( QgsFileWidget::SaveFile );
   mOutputLayer->setDialogTitle( tr( "Enter result file" ) );
   mOutputLayer->setDefaultRoot( settings.value( QStringLiteral( "/RasterCalculator/lastOutputDir" ), QDir::homePath() ).toString() );
   connect( mOutputLayer, &QgsFileWidget::fileChanged, this, [ = ]() { setAcceptButtonState(); } );