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GH-37488: [C++] Disable unity build for Azure SDK for C++ (#37489)

### Rationale for this change

It seems that Azure SDK for C++ doesn't support unity build.

### What changes are included in this PR?

Force to disable unity build.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: #37488

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>