Constant ENV_BASE_CONFIG 

pub const ENV_BASE_CONFIG: &str = "ONIRO_BASE_CONFIG";

Path to the base RunConfig TOML the trial config is layered on top of.

This is the text of oniro_cli default-config (or a user-supplied base), captured once by the harness; the child reads it, applies the sampled overlay values at their dotted keys, and writes the merged trial config.