arrow

GH-38183: [CI][Python] Use pipx to install GCS testbench (#43852)

### Rationale for this change

Installing the GCS testbench using the same Python that's being used to test PyArrow is fragile: some testbench versions may not be compatible, or there could be conflicts among the dependencies of the respective libraries.

### What changes are included in this PR?

Use `pipx` to install the GCS testbench in a separate, controlled environment, using an appropriate Python version.

### Are these changes tested?

Yes, by CI.

### Are there any user-facing changes?

No.

* GitHub Issue: #38183

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>