GH-51345: [CI] Fix pipx install for packaging on Debian

[tadeja]

Rationale for this change

Fix #51345 uninstalling packaging 25.0 fails with error: uninstall-no-record-file
(new pipx 1.17.1 requires packaging>=26 )

What changes are included in this PR?

Add --ignore-installed flag to successfully install pipx

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

• GitHub Issue: [CI][Docs] debian-cpp job fails on pip install packaging #51345

[tadeja]

@github-actions crossbow submit test-debian-13-docs test-debian-13-cpp-amd64 test-fedora-42-cpp

[github-actions]

Revision: 2af9626

Submitted crossbow builds: ursacomputing/crossbow @ actions-51f3ab7de1

Task	Status
test-debian-13-cpp-amd64
test-debian-13-docs
test-fedora-42-cpp

[github-actions]

⚠️ GitHub issue #51345 has been automatically assigned in GitHub to PR creator.

[tadeja]

@github-actions crossbow submit test-conda-cpp

[Copilot]

🟢 Approval recommended

No unresolved review issues were identified.

Pull request overview

Updates the CI GCS testbench installer to support Debian-managed Python packages when installing pipx.

Changes:

• Adds --ignore-installed to the pipx installation command.

File summaries

File	Description
ci/scripts/install_gcs_testbench.sh	Adjusts pipx installation for Debian compatibility.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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[github-actions]

Revision: 2af9626

Submitted crossbow builds: ursacomputing/crossbow @ actions-2e346766eb

Task	Status
test-conda-cpp

[tadeja]

@raulcd I've also tried a fix with venv but actually using --ignore-installed from #51335 makes more sense here. As PR #51335 is not split yet, we can use this one.