fix: add allow-downloads to iframes

[keyz]

What kind of change does this pull request introduce?

Add the sandbox="allow-downloads" attribute to iframes. See codesandbox/codesandbox-client#8253 for more context.

I'm not 100% sure whether this is the right fix, however:

• In production, the iframe on https://codesandbox.io/p/sandbox/allow-downloads-repro-py27jv contains the allow="accelerometer; camera; encrypted-media; geolocation; gyroscope; hid; microphone; midi; clipboard-write;" attribute.
• The string clipboard-write is nowhere to be found in the codesandbox-client repo. But the value of the attribute literally matches how sandpack sets it.
• Does it mean sandpack is actually used in production to power sandboxes, therefore adding allow-downloads here will fix the regression for preview sandboxes? 🤔

What is the current behavior?

No allow-downloads (codesandbox/codesandbox-client#5936), which had been fixed by codesandbox/codesandbox-client#5163 and codesandbox/codesandbox-client#6384 but regressed again.

What is the new behavior?

If I understand correctly, updating the code here should fix the issue for sandbox previews (e.g. https://codesandbox.io/p/sandbox/allow-downloads-repro-py27jv).

The new list of sandbox permissions matches the behavior in https://github.com/codesandbox/codesandbox-client/blob/45862c7b9c382abce725f1272c3aed560162776a/packages/common/src/components/Preview/index.tsx#L648

What steps did you take to test this? This is required before we can merge, make sure to test the flow you've updated.

Not sure how to let codesandbox pick up the change here, but unit tests pass. Let me know if I'm on the right path!

Checklist

• Documentation;
• Storybook (if applicable);
• Tests;
• Ready to be merged;

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