Fix linux/arm64 CI image build broken by ibm-db2 provider

[FrankYang0529]

ibm-db has no Linux aarch64 wheel and its sdist fails to build there, so every linux/arm64 CI image build has failed since the provider was merged. That blocks the ARM test on main. It shows NameError: name 'arch_' is not defined.

CI Run Examples:

• https://github.com/apache/airflow/actions/runs/32703122015/job/97358672625
• https://github.com/apache/airflow/actions/runs/32661777905/job/97249003926

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Was generative AI tooling used to co-author this PR?

• Yes - Claude Code

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