Add step start time to `ViewStepLog` (#24980)

Part of #24876

According to
https://github.com/go-gitea/gitea/pull/24876#discussion_r1205565622 , we
need the start time of the step to get the log time offset.

routers/web/repo/actions/view.go

@@ -109,6 +109,7 @@ type ViewStepLog struct {
 	Step    int                `json:"step"`
 	Cursor  int64              `json:"cursor"`
 	Lines   []*ViewStepLogLine `json:"lines"`
+	Started int64              `json:"started"`
 }
 
 type ViewStepLogLine struct {
@@ -244,6 +245,7 @@ func ViewPost(ctx *context_module.Context) {
 				Step:    cursor.Step,
 				Cursor:  cursor.Cursor + int64(len(logLines)),
 				Lines:   logLines,
+				Started: int64(step.Started),
 			})
 		}
 	}