change Job recievier type from Job to *Job.

build method of Job modify outputTs.
if receiver type is Job, it will modify copied Job and
doesn't affect caller data.
diff --git a/worker.go b/worker.go
index 7b8d723..b02c573 100644
--- a/worker.go
+++ b/worker.go
@@ -178,7 +178,7 @@
 	return err
 }
 
-func (j Job) createRunners() []runner {
+func (j *Job) createRunners() []runner {
 	runners, _ := j.ex.createRunners(j.n, false)
 	return runners
 }
@@ -192,7 +192,7 @@
 	return st.ModTime().Unix()
 }
 
-func (j Job) build() {
+func (j *Job) build() {
 	if j.n.IsPhony {
 		j.outputTs = -2 // trigger cmd even if all inputs don't exist.
 	} else {