Right now (when #78113 passes) any custom device has TargetPlatform.linux_arm64, which is okay for now since only platform-agnostic stuff is built (the assets bundle).

In the future, it'd be better to allow configuring or auto-detecting the target platform. If we at some point want to build release artifacts for custom devices, we need to know the target arch.

(This is just an issue so I have something for the TODO-item in the PR-code to link to)