Worker<Sandbox> presumes the process is running in the same filesystem as sandbox

[itegulov]

#220 presumes that the sandbox instance is running on the same filesystem as the current process. Specifically, this tries to pull the validator keys from the local filesystem, while the sandbox process might be running in a virtualized container or remotely.

We could either provide a configuration parameter for NetworkBuilder presuming that the user can fetch the keys themselves or add a sandbox endpoint that returns the keys in plaintext. Not sure if there is anything easier/more convenient.