setup_builder: retry transient sticky disk requests

[lukebjerring]

Builder setup can fail before BuildKit starts when the sticky disk request crosses a brief service disruption. One observed job received:

failed to expose and mount sticky disk: unexpected status code: 503

The action now retries only clearly transient unavailable, 502, 503, and 504 responses. It does not retry 500 or deadline errors that may have completed part of the expose operation. The existing 45 second setup deadline still bounds all attempts.

Tests cover recovery from a 503 and immediate failure for a 500. CI must rebuild the checked-in dist/ bundle because the private Buf package is not available outside the repository workflow.

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[lukebjerring]

This issue is already fixed, the test is just using a really old setup