Fix ECE button size in shortcode cart page

[Mayisha]

Fixes #3989

Changes proposed in this Pull Request:

• Added flex: 1 and min-width so that each ECE button takes up the available space in the container.

Testing instructions

• Enable Google/Apple Pay, Link and Amazon Pay.
• Add a product to your cart and go to the shortcode cart page.
• In develop notice that the buttons are not taking full width.

• In this branch, confirm that the buttons are full width and aligned with the proceed to checkout button.

• Confirm that the buttons are rendering nicely in both shortcode cart and checkout pages for different window width.

[hsingyuc]

@Mayisha, can you confirm whether you also see the gaps between the ECE buttons in Blocks cart page in Safari?