miner: actually sign aggregate transaction inputs

[DaMatrix]

Description

Previously, aggregation transactions never actually got signed, and were being submitted to the mempool with an empty redeem script.

Changelog

• List the changes to the codebase that this PR introduces

Type of Change

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• Bug fix
• New feature
• Enhancement or optimization
• Documentation update
• Other (please specify)

Checklist

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• I have tested the changes locally and they work as expected.
• I have added necessary documentation or updated existing documentation.
• My code follows the project's coding standards and style guidelines.
• I have added/updated relevant tests to ensure the changes are properly covered.
• I have checked for and resolved any merge conflicts.
• My commits have clear and descriptive messages.

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Related Issues (if applicable)

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