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MINOR: [CI][R] Increase timeout for ubuntu-r-valgrind (#44316)

### Rationale for this change

test-r-linux-valgrind has been failing consistently due to timeout since September 20th.
See [first failure](https://github.com/ursacomputing/crossbow/actions/runs/10951024478/job/30407361495) and [last failure](https://github.com/ursacomputing/crossbow/actions/runs/11206537690/job/31147433866)

### What changes are included in this PR?

Increase timeout

### Are these changes tested?

Will validate via archery

### Are there any user-facing changes?

No

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