Wildcard V2: Add label hover style to `BaseControlInput`

Created by: courier-new

Adds the missing hover style that darkens the text to $gray-12 to the label for BaseControlInput as featured in the Wildcard design files, which controls radio buttons and checkboxes:

https://user-images.githubusercontent.com/8942601/153955776-0de79914-2a92-4522-9cd5-ec41a8b35a80.mov

It's subtle but it's something.

"Valid" coloration (green for valid, red for invalid) overwrites this hover style.

Test plan

Verified visually in Storybook for both the checkbox and radio button components.