getDefinitions(additionalEnv) {
        return Object.fromEntries(
            Object.entries({ ...this.env, ...additionalEnv }).map(([key, value]) => {
                return [`process.env.${key}`, JSON.stringify(value)];
            })
        );
    }
Build all MIX_ definitions for Webpack's DefinePlugin.
This is no longer used but here for backwards compat.

@deprecated
@param {Record<string, string>} additionalEnv