Icon margin on select menu doesn't match designs

[GCHQ-Developer-847]

Summary of the bug

When an icon is displayed on a menu option in an ic-select, the margin on the right of the icon is 2px but it should be 4px, based on the Figma designs.

🪜 How to reproduce

1. Go to the "Custom components" select story.
2. Inspect the icons on the menu options and see that the margin uses the --ic-space-xxs variable (i.e. 2px)

📸 Screenshots or code

Figma:

🧐 Expected behaviour

The margin-right should use the 4px / 0.125rem CSS spacing variable to match the designs.