CI: fix empty Windows.zip release artifact

For the zip command without the -r option, Windows.zip
on release contained empty folders.

Signed-off-by: Raluca Groza <[email protected]>

Author: ccraluca

CI/azure/prepare_assets.sh

@@ -44,7 +44,7 @@ release_artifacts() {
         done
 	cp /home/vsts/work/1/s/CI/azure/README.txt ./Windows
 	cd Windows
-	zip Windows.zip ./*
+	zip -r Windows.zip ./*
 	cp ./Windows.zip ../
 	cd ..
 	rm -r Windows