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GH-43618: [Packaging][Python] Fix vcpkg version detection in macOS wheel build jobs (#43615)

### Rationale for this change

`.env` is a shell script. We can use `VCPKG=... # comment` form in a shell script but our handmade `VCPKG` value in `.env` detection doesn't care about the case.

### What changes are included in this PR?

Use shell instead of handmade detection.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #43618

Lead-authored-by: Sutou Kouhei <kou@clear-code.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>