function env( opts ) {
	var out = copy();

	out.WORKER_CMD = opts.cmd;
	out.WORKER_ENCODING = opts.encoding;
	out.WORKER_MAX_BUFFER = opts.maxBuffer;

	if ( opts.ordered ) {
		out.WORKER_ORDERED = '1';
	}
	if ( opts.uid ) {
		out.WORKER_UID = opts.uid;
	}
	if ( opts.gid ) {
		out.WORKER_GID = opts.gid;
	}
	return out;
}
Returns worker environment variables.

@private
@param {Options} opts - options
@param {string} opts.cmd - executable file/command
@param {boolean} opts.ordered - boolean indicating whether to preserve order of script output
@param {(NonNegativeInteger|null)} opts.uid - process user identity
@param {(NonNegativeInteger|null)} opts.gid - process group identity
@param {string} opts.encoding - `stdio` encoding
@param {NonNegativeInteger} opts.maxBuffer - max child process `stdio` buffer size
@returns {Object} environment variables