tests: Turn off GHCR mirror by default, fix build output

[def-]

Two reports of slow GHCR locally: https://materializeinc.slack.com/archives/C01LKF361MZ/p1762792334347149

Stays enabled in CI and for people setting MZ_GHCR=1 manually.

(This will conflict with my own change that cleans up some MZ_GHCR handling too: #34092, but it's unfortunately not green in CI yet)

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[ggevay]

Thank you!