Box64: add libstdc++6 + update package name

[QuintenQVD0]

• This update the package name, and the way the KEY is downloaded.
• add libstdc++6 as it is needed for ragemp

Description

All Submissions:

• [x] Have you ensured there aren't other open Pull Requests for the same update or change?
• [x] Have you created a new branch for your changes and PR from that branch and not from your master branch?