Improve UX for running against a pre-booked device

After we Make SSH toolchain an optional override (#87 - closed), we should improve the UX so that users don't have to specify --extra_toolchains manually every time.

Ideally, we'd be able to infer it from any @rules_labgrid//labgrid/flag/device flag being used:

bazel test :test --@rules_labgrid//labgrid/flag/device:address=<ip>

If this isn't possible, we could settle for something like this:

bazel test :test --@rules_labgrid//labgrid/flag/device --@rules_labgrid//labgrid/flag/device:address=<ip>

This should pass the SSH and ADB toolchains as extra toolchains, and the OS constraint would take care of selecting the right one to use.