Avoid use of aspect-ratio for prominent simulator run button

[microbit-matt-hillsdon]

From photos on twitter, some folks are using old enough Edge that they don't have aspect-ratio (pre v88).

We only use it for the run button and can probably avoid it so let's try that at some point.

Not urgent, it's just a flat oval and there may well be other issues with a browser that old too.

[microbit-matt-hillsdon]

Also affects Safari 13.