Replace `bazel --batch` with `bazel shutdown`
Replace `bazel --batch` with `bazel shutdown`

Fixes this warning:
--batch mode is deprecated. Please instead explicitly shut down your Bazel
server using the command "bazel shutdown".

RJPercival/trillian,google/trillian,AlCutter/trillian,gdbelvin/trillian,google/trillian,gdbelvin/trillian,AlCutter/trillian,gdbelvin/trillian,RJPercival/trillian