Variable font output filename doesn't use designspace info

[ryanbugden]

It looks like the output filename for a variable font uses the interpolable operator name instead of the VariableFontDescriptor’s filename attribute.

https://github.com/typemytype/batchRoboFontExtension/blob/be2fd5178a5019aa4fc0c5dd1f6b456338dc5135/source/lib/batchGenerators/variableFontsGenerator/init.py#L497-L502