Fix an actions schedule bug (#28942) (#28999)

Backport #28942 by @Zettat123

In #28691, schedule plans will be deleted when a repo's actions unit is
disabled. But when the unit is enabled, the schedule plans won't be
created again.

This PR fixes the bug. The schedule plans will be created again when the
actions unit is re-enabled

---------

Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/actions/workflows.go

@@ -146,6 +146,41 @@ func DetectWorkflows(
 	return workflows, schedules, nil
 }
 
+func DetectScheduledWorkflows(gitRepo *git.Repository, commit *git.Commit) ([]*DetectedWorkflow, error) {
+	entries, err := ListWorkflows(commit)
+	if err != nil {
+		return nil, err
+	}
+
+	wfs := make([]*DetectedWorkflow, 0, len(entries))
+	for _, entry := range entries {
+		content, err := GetContentFromEntry(entry)
+		if err != nil {
+			return nil, err
+		}
+
+		// one workflow may have multiple events
+		events, err := GetEventsFromContent(content)
+		if err != nil {
+			log.Warn("ignore invalid workflow %q: %v", entry.Name(), err)
+			continue
+		}
+		for _, evt := range events {
+			if evt.IsSchedule() {
+				log.Trace("detect scheduled workflow: %q", entry.Name())
+				dwf := &DetectedWorkflow{
+					EntryName:    entry.Name(),
+					TriggerEvent: evt,
+					Content:      content,
+				}
+				wfs = append(wfs, dwf)
+			}
+		}
+	}
+
+	return wfs, nil
+}
+
 func detectMatched(gitRepo *git.Repository, commit *git.Commit, triggedEvent webhook_module.HookEventType, payload api.Payloader, evt *jobparser.Event) bool {
 	if !canGithubEventMatch(evt.Name, triggedEvent) {
 		return false