fix: evaluate schedules in the cronjob's spec.timeZone (#428)

Schedules were always evaluated in UTC because `schedule.Next` uses the location of `lastScheduleTime`, which the API returns in UTC. A CronJob with `spec.timeZone` set was therefore reported as not running every day. For example, Paidit's `prod/dayclose` runs at `0 7 * * *` Europe/Stockholm: it ran at 05:00Z, but cron-checker expected 07:00Z and alerted every minute from 09:02 local time.

- Convert `since` to the CronJob's `spec.timeZone` before computing the next run. CronJobs without a `timeZone` behave as before.
- An invalid time zone returns an error, the same way an invalid schedule does.
- Embed `time/tzdata`, because the image is `FROM scratch` and has no zoneinfo.
- Tests: an invalid time zone, and a time-zone case that fails without the fix.
- Bumped golang.org/x/net to v0.60.0. govulncheck was already failing on main with five x/net advisories, which blocks the build job.
- Bumped the go toolchain to 1.27.2 for eight stdlib advisories. This is the same change as Renovate #427, which will close once this is merged.

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https://claude.ai/code/session_01C8SW6vsbkrTKjLNatQ8JTg
Reviewed-on: https://gitea.unbound.se/unboundsoftware/cron-checker/pulls/428

main.go

@@ -8,6 +8,7 @@ import (
 	"os/signal"
 	"syscall"
 	"time"
+	_ "time/tzdata" // the image is FROM scratch, so embed zoneinfo for spec.timeZone
 
 	"github.com/alecthomas/kingpin/v2"
 	"github.com/multiplay/go-slack/chat"
@@ -19,8 +20,10 @@ import (
 	"k8s.io/client-go/rest"
 )
 
-var checkFunc = doCheck
-var exitFunc = os.Exit
+var (
+	checkFunc = doCheck
+	exitFunc  = os.Exit
+)
 
 func main() {
 	slackUrl := kingpin.Flag("slack-url", "The Slack Webhook URL").Envar("SLACK_URL").Required().String()
@@ -68,6 +71,14 @@ func doCheck(client Client, slackUrl string, ic chan os.Signal, sleepTime time.D
 					if c.Status.LastScheduleTime != nil {
 						since = *c.Status.LastScheduleTime
 					}
+					// The schedule is evaluated in the location of since, so use the cronjob's time zone (UTC if unset)
+					if c.Spec.TimeZone != nil {
+						loc, err := time.LoadLocation(*c.Spec.TimeZone)
+						if err != nil {
+							return fmt.Errorf("error loading time zone of %s/%s (%s): %w", c.Namespace, c.Name, *c.Spec.TimeZone, err)
+						}
+						since = v1.NewTime(since.In(loc))
+					}
 					schedule, err := parser.Parse(c.Spec.Schedule)
 					if err != nil {
 						return fmt.Errorf("error parsing schedule of %s/%s (%s): %w", c.Namespace, c.Name, c.Spec.Schedule, err)