Fix legend URL for difference item generated using difference tool

[na9da]

Reported in datacube slack.

When diffing two WMS dates, in the difference mode the correct legend for difference output is shown. But when exiting the difference mode, the difference output in the workbench shows the wrong legend.

To generate the correct legend for diff mode, the time parameter must be set to date1,date2. Example: https://ows.dev.dea.ga.gov.au/wms?service=WMS&version=1.1.0&request=GetLegendGraphic&format=image%2Fpng&transparent=True&layer=ga_s2ls_intertidal_cyear_3&styles=intertidal_elevation_adaptive&time=2016-01-01%2C2022-01-01

Test dataset: https://terria-cube.terria.io/#share=s-weAnFnIaphPxgUQRnDa1FftkPom

correct legend:

incorrect legend: