Closes https://core.trac.wordpress.org/ticket/63373

Follow-up on #64770 and #69651

What?

This PR changes the style selector for buttons to this:

// From
.wp-block-button__link {
  width: 100%;
}
// To
.wp-block-buttons .wp-block-button__link {
  width: 100%;
}

Why?

The width: 100% style is needed to stretch the buttons in the vertical layout. However, the .wp-block-button__link selector resulted in forcing the button's width to 100% width even though the button was not a child of a flex layout, such as in the following HTML:

<p class="wp-block-button">
  <a class="wp-block-button__link" />
</p>

This problem occurred in the Post Comments Form block, so we fixed the problem in #69651. However, it's possible that consumers are using this button selector in other places as well.

How?

Apply 100% width style to only the button links inside the buttons.

Testing Instructions

• Insert a Buttons block.
• Change the justification to "Stretch items" and change the orientation to "Vertical".
• Confirm that buttons are correctly stretched.

• Insert a Post Comments Form block.
• Confirm that the submit button isn't 100% width.