Bug: Additional border attached to base copy Button with `tertiary` variant

### Steps to reproduce

1. Go to the Copy Button component
2. Enable dark mode
3. Enable one of states: `hover, focus, active`

### Expected Behavior

Should not add an additional border to the button

### Actual Behavior

Additional border attached to the button;
[Reference](https://github.com/dequelabs/cauldron/pull/1608#issuecomment-2312421473)

### Version

latest

### What browsers are you experiencing the problem on?

_No response_

### Relevant log output

_No response_

### Anything else we should know?

_No response_