Move Zephyr SDK Build Into Docker Image for Zephyr CI Jobs

[BujSet]

🚀 The feature, motivation and pitch

Right now, for zephyr CI jobs, the Zephyr SDK is downloaded and installed when a job runs. This eats into the CI jobs timeout limit, when it doesn't relate to functional correctness of the job. Ideally, the Zephyr SDK should be downloaded when the Zephyr docker image builds, which will save time and make the Zephyr docker image more useful for future CI jobs that have longer examples.

Alternatives

No response

Additional context

Refer to this PR for context on a Zephyr CI Jobs.

RFC (Optional)

No response