get address for DICE artifacts from config

Our prototype currently hard codes these addresses in the source code. Getting them from the chip.toml at build time would be better. At first look doing so would require adding some plumbing to xtask. The data we would need is in the 'chip' key from the top level toml table but AFAIK the currently available methods for getting at the app toml exposed to tasks at build time aren't sufficient:

• build_util::config exposes the app config that includes tables under [config]: https://github.com/oxidecomputer/hubris/blob/09fb9271df0208dc571ade28feb0503c78707817/build/util/src/lib.rs#L50
• build_util::task_config exposes tables under the [task_name] table: https://github.com/oxidecomputer/hubris/blob/09fb9271df0208dc571ade28feb0503c78707817/build/util/src/lib.rs#L57