"headers" not respected for embedded Sandbox

[colinrobertbrooks]

Issue Description

When passing the headers option to an embedded Sandbox, the supplied headers are not set in the UI. Potentially related to #7685.

Link to Reproduction

https://codesandbox.io/p/devbox/nostalgic-chebyshev-shhcqq

Reproduction Steps

• Run yarn start
• In the embedded sandbox, open the "Headers" tab and see the headers have not been set
• In the options, set embed: false
• In the linked sandbox, open the "Headers" tab and see the headers have been set