Use UTC as default timezone when schedule Actions cron tasks (#31742)

Fix #31657.

According to the
[doc](https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#onschedule)
of GitHub Actions, The timezone for cron should be UTC, not the local
timezone. And Gitea Actions doesn't have any reasons to change this, so
I think it's a bug.

However, Gitea Actions has extended the syntax, as it supports
descriptors like `@weekly` and `@every 5m`, and supports specifying the
timezone like `TZ=UTC 0 10 * * *`. So we can make it use UTC only when
the timezone is not specified, to be compatible with GitHub Actions, and
also respect the user's specified.

It does break the feature because the times to run tasks would be
changed, and it may confuse users. So I don't think we should backport
this.

## ⚠️ BREAKING ⚠️

If the server's local time zone is not UTC, a scheduled task would run
at a different time after upgrading Gitea to this version.

models/actions/schedule_spec.go

@@ -5,6 +5,8 @@ package actions
 
 import (
 	"context"
+	"strings"
+	"time"
 
 	"code.gitea.io/gitea/models/db"
 	repo_model "code.gitea.io/gitea/models/repo"
@@ -32,8 +34,29 @@ type ActionScheduleSpec struct {
 	Updated timeutil.TimeStamp `xorm:"updated"`
 }
 
+// Parse parses the spec and returns a cron.Schedule
+// Unlike the default cron parser, Parse uses UTC timezone as the default if none is specified.
 func (s *ActionScheduleSpec) Parse() (cron.Schedule, error) {
-	return cronParser.Parse(s.Spec)
+	parser := cron.NewParser(cron.Minute | cron.Hour | cron.Dom | cron.Month | cron.Dow | cron.Descriptor)
+	schedule, err := parser.Parse(s.Spec)
+	if err != nil {
+		return nil, err
+	}
+
+	// If the spec has specified a timezone, use it
+	if strings.HasPrefix(s.Spec, "TZ=") || strings.HasPrefix(s.Spec, "CRON_TZ=") {
+		return schedule, nil
+	}
+
+	specSchedule, ok := schedule.(*cron.SpecSchedule)
+	// If it's not a spec schedule, like "@every 5m", timezone is not relevant
+	if !ok {
+		return schedule, nil
+	}
+
+	// Set the timezone to UTC
+	specSchedule.Location = time.UTC
+	return specSchedule, nil
 }
 
 func init() {