Function apply_env_overrides 

fn apply_env_overrides(
    env: &mut HashMap<String, String>,
    overrides: &HashMap<String, String>,
)

Layer per-spawn env overrides over the account env: a present key replaces, an absent key keeps, and a key mapped to an empty string removes the variable entirely - removal is what makes an overridden-away credential actually absent rather than present-but-blank.