CI: Set secret_detection job's dependencies and needs to empty

In the current CI pipeline, the secret_detection job is in the Test
stage, which is after the Build stage. The jobs in the Test stage will
download the artifacts of the Build stage by default, and these
artifacts are unnecessary for the secret_detection job. So set
dependencies as empty to the secret_detection job to avoid
downloading artifacts of the Build stage.

Also, the secret_detection job doesn't necessarily need to be executed
after the Build stage, so set needs to null as well.

Signed-off-by: Robbie Cao <robbie.cao@arm.com>