Build: Fix release badge on repros

[yannbf]

Closes #

What I did

The README of our reproductions contains a badge about its versions, and currently it shows 'main' instead of 'latest' (when applicable):

<img
  alt="Storybook main Badge"
  src="https://img.shields.io/npm/v/@storybook/react/main"
/>

I also fixed a similar thing for the stackblitz URL

How to test

1. The generated README of sandbox repo should be updated

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]