Edit: An earlier version of this answer was incorrect, and has been updated.
You can't specify the country code of your relay since your relay does not publish this. Instead, other tor clients/relays derive this information from your relay's IPv4 address. When a client/relay downloads a copy of the consensus, it will look up each relay's country code in its local geoip database. This geoip map from IP addresses to country codes is shipped as part of tor's source code and since it is not possible to modify this map for other relays/clients, you cannot modify the country code that they assign to you.
When a client/relay downloads a consensus, it build/updates its
nodelist_set_consensus(ns). Within this function, it calls
node_set_country(node) to update the country code for each node in the consensus. This is where the lookup in the geoip database occurs.
If you're running a local Chutney network and you have control of every client/relay, you can try modifying each of their geoip files to your own custom maps. Tor/Chutney don't support assigning country codes, but here's an idea: I haven't tried this, you could try modifying chutney to bind each relay's OR port on a different local address (for example the first node on 127.0.0.1, the second on 127.0.0.2, etc), and modify the geoip map to map these addresses to different countries. For example replace the contents of the geoip file with the values:
where 2130706433 is the decimal representation of 127.0.0.1, etc.