bin/get-pick-list.sh: fix redirection in sh

"&>" is bash specific.

Fixes: e0dbfc99 ("bin/get-pick-list.sh: warn when commit lists invalid sha")
Cc: Juan A. Suarez <jasuarez@igalia.com>
Cc: Eric Engestrom <eric.engestrom@intel.com>
Cc: Dylan Baker <dylan@pnwbakers.com>
Cc: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
(cherry picked from commit 3ec9ab80)