GH-46843: [C++] Don't use unity build for bundled AWS SDK for C++

[kou]

Rationale for this change

AWS SDK for C++ related products don't support unity build.

What changes are included in this PR?

Always disable unity build only for AWS SDK for C++ related products.

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

• GitHub Issue: [C++] Bundled AWS SDK for C++ can't be built with unity build #46843

[github-actions]

⚠️ GitHub issue #46843 has been automatically assigned in GitHub to PR creator.

[raulcd]

@github-actions crossbow submit test-debian-12-docs

[github-actions]

Revision: daccbbe

Submitted crossbow builds: ursacomputing/crossbow @ actions-6d8783d78f

Task	Status
test-debian-12-docs

[raulcd]

+1 this is fixing our current issue. Is AWS SDK supposed to support it? Should we open an issue upstream?

[kou]

I think that AWS SDK for C++ doesn't care about unity build. So it doesn't support unity build isn't strange.
So we don't need to open an issue to upstream.

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 3 benchmarking runs that have been run so far on merge-commit 5f0aeb5.

There were 119 benchmark results with an error:

• Commit Run on amd64-m5-4xlarge-linux at 2025-06-20 01:38:26Z
  • tpch (R) with engine=arrow, format=parquet, language=R, memory_map=False, query_id=TPCH-04, scale_factor=10
  • tpch (R) with engine=arrow, format=native, language=R, memory_map=False, query_id=TPCH-12, scale_factor=1
• and 117 more (see the report linked below)

There were no benchmark performance regressions. 🎉

The full Conbench report has more details.