[config] chore: move envelope API call to fetch in parallel rather than sequentially

[jasonbarry]

https://github.com/netlify/build/blob/main/packages/config/src/api/site_info.ts#L46-L59

Once all sites have migrated to the new environment variables experience, it will be safe to move this call to the Promise.all() call above it. Should make things a little bit faster for the bootup of ntl dev, ntl env, and any other commands that use the cached config's siteInfo.

[jasonbarry]

Blocked by https://github.com/netlify/bitballoon/issues/16952