Incompatible with nix on macOS (Darwin)

At least due to the dependency on the Shadow package (shadow-utils) which is specific to Linux to manage user accounts, groups, and secure password files. Since Darwin's architecture for managing users, groups and passwords is different from Linux's a direct port of shadow-utils for Darwin does not seem to exist.

I suggest to update the Readme to reflect that jailed-agents is unfortunately limited to Nix on Linux (for now?)