feat: make SSH toolchain an override
This allows using the SSH toolchain to override the labgrid config toolchain resolution, while honouring the target platform. For example, the following will run against an arm64
device via SSH:
bazel test :test \
--platforms=@toolchain_utils//toolchain/platform:arm64-linux \
--extra_toolchains=@rules_labgrid//labgrid/toolchain/config/ssh \
--@rules_labgrid//labgrid/flag/device:address=$BOARD_IP
Closes #87 (closed).