fix: use correct committee validator bc1[1] for attestation2

[Bilogweb3]

Copy-paste bug: attestation2 created from committee bc1[1] was added to pool with validator from bc0[1] instead.
Test verifies cross-committee aggregation, so correct committee assignment is critical for test validity.

[github-actions]

Pull requests to the stable branch are not allowed.
Please open your pull request against the unstable branch.
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[tersec]

Feel free to reopen this against unstable; it looks like a good fix.