Nix by default only polls for user input when confirming the values,
in a CI environment there's no way to progress the builds without passing
the '--accept-flake-config' value explicitly.

See: https://github.com/NixOS/nix/issues/7071

https://buildkite.com/comm/tunnelbroker-unit-tests/builds/50#01835b9d-1109-4c97-9c3a-444f70fb6a82 passes