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GH-40706: [CI][Python] Activate ARROW_PYTHON_VENV if defined in sdist-test job (#40707)

### Rationale for this change

[python-sdist](https://github.com/ursacomputing/crossbow/actions/runs/8355876590/job/22871821900) is failing.

### What changes are included in this PR?

Activate `ARROW_PYTHON_VENV` on `python_sdist_test.sh`

### Are these changes tested?

Via archery

### Are there any user-facing changes?

No
* GitHub Issue: #40706

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