GH-51751: [CI][Packaging] Fix formula path for homebrew-cpp job

[tadeja]

Rationale for this change

Fix #51751
homebrew-core's formulae got letter-folders in 2023 but our macro from commit 5ad5ddc kept copying to top-level Formula/ hence homebrew-cpp job tested homebrew-core's formulae instead of both apache-arrow.rb and apache-arrow-glib.rb in dev/tasks/homebrew-formulae/

What changes are included in this PR?

Copy to Formula/a/ and
change to formula_opt_prefix("llvm") per audit check

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

Was AI used for this PR?

PR code and description written by:

• Human
• AI

Reviewed before submission by:

• Human
• AI
• Not reviewed

• GitHub Issue: [CI][Packaging] Nightly job homebrew-cpp fails to find uriparser #51751

[tadeja]

@github-actions crossbow submit homebrew-cpp

[github-actions]

⚠️ GitHub issue #51751 has been automatically assigned in GitHub to PR creator.

[github-actions]

Revision: eac0f60

Submitted crossbow builds: ursacomputing/crossbow @ actions-9f8e923d8f

Task	Status
homebrew-cpp

[tadeja]

With the path fix the job now tests our formula, so the new brew audit check finds this:
https://github.com/ursacomputing/crossbow/actions/runs/37315746512/job/111781924487#step:4:3644

Error: 1 problem in 1 formula detected.
  * line 74, col 21: Use `formula_opt_prefix("llvm")` instead of `Formula["llvm"].opt_prefix`.

This was already updated June in Homebrew/homebrew-core@208ed01

[tadeja]

@github-actions crossbow submit homebrew-cpp

[github-actions]

Revision: ba3ea28

Submitted crossbow builds: ursacomputing/crossbow @ actions-dda22448f3

Task	Status
homebrew-cpp

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused changes correctly address the reported CI failure and Homebrew audit requirement.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes the Homebrew CI job to test Arrow’s local formulae rather than homebrew-core versions.

Changes:

• Copies Arrow formulae into Homebrew’s letter-based Formula/a/ directory.
• Uses the audit-compliant LLVM prefix helper.

File	Description
dev/​tasks/​macros.jinja	Corrects the Homebrew formula destination.
dev/​tasks/​homebrew-formulae/​apache-arrow.rb	Updates LLVM prefix resolution.

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[kou]

+1