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GH-36402: [CI][macOS] Ignore `brew update` failure (#36403)

### Rationale for this change

It's better that we always use the latest Homebrew formulae but it sometimes fails. It's better that we run our tests with old Homebrew formulae than our CI jobs entirely failed.

### What changes are included in this PR?

Ignore `brew update` failure.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: #36402

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