Grouped attached buttons seem to have thick shared borders

[microbit-grace]

Chakra for comparison:

Python editor for comparison:

[microbit-matt-hillsdon]

I have recently aligned them with Chakra which is a -1px border overlap for the grouped buttons.

But that still gives 3px overlap for 2px border buttons which I do think looks a bit odd.

The effect in the Python Editor is at the call site and always was, but perhaps we should pull it in here. Might be a bit awkward because might depend on the button variant / border width..