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GH-36477: [CI][macOS] Ignore brew update failure on crossbow tasks (#36478)

### Rationale for this change

Some nightly verification tasks are failing.

### What changes are included in this PR?

Ignore possible errors on `brew update` commands on crossbow tasks.

### Are these changes tested?

Crossbow

### Are there any user-facing changes?

No
* Closes: #36477

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