added -nographics flag to build.sh (#166)

Author: BLaZeKiLL

action/steps/build.sh

@@ -111,6 +111,7 @@ echo ""
 xvfb-run --auto-servernum --server-args='-screen 0 640x480x24' \
   /opt/Unity/Editor/Unity \
     -batchmode \
+    -nographics \
     -logfile /dev/stdout \
     -quit \
     -customBuildName "$BUILD_NAME" \