Button with `scheme: :invisible` has different font colors

[HDinger]

Version

0.19.0

Steps to reprdocue

• Use an Primer::Beta::Button with scheme: :invisible
• Compare how the button looks w/o a trailing or leading visual
• The issue can be seen here: https://view-components-storybook.eastus.cloudapp.azure.com/view-components/lookbook/inspect/primer/beta/button/invisible_all_visuals

Actual behaviour

• Without an additional visual, the button font is blue
• With an additional visual it is black

Expected behaviour

• The font color should always be the same
• I assume, the default should be black according to this specification: https://primer.style/components/button#invisible