fix: only enable rosetta by default when installed

[manojmahapatra]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Addressing #1136

On fresh installs, build.rosetta defaults to true, which makes the builder VM request rosetta even when it isn’t installed. That looks like hits a potential Virtualization.framework install‑prompt bug and fails the build unexpectedly. This change keeps the existing auto install behavior for explicit opt‑in while preventing the default path from enabling Rosetta unless it’s already installed.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

[jglogan]

Hi @manojmahapatra, please setup signed commits. See: https://github.com/apple/containerization/blob/main/CONTRIBUTING.md#pull-requests

[manojmahapatra]

Yes, it was missing on my new MBP, I've set it up correctly now.

[manojmahapatra]

@jglogan when you've time, can you please review this?

[jglogan]

@jglogan when you've time, can you please review this?

@manojmahapatra Sorry I have been swamped and will continue to be through next week.

I did get a chance to look at a cluster of PRs tonight. This looks perfectly fine. We've still got a few lingering container lifecycle bugs that hit us nondeterministically in CI, so I'm trying to get a build working and then will approve/merge. Apologies for the delay!

[manojmahapatra]

No worries, and no rush btw. I just wanted to bubble this up. Thank you!