serve ftp: ensure modtime is passed as UTC always to fix timezone oddities

See: https://forum.rclone.org/t/ftp-server-reports-file-timestamps-in-utc/26274

cmd/serve/ftp/ftp.go

@@ -14,6 +14,7 @@ import (
 	"os/user"
 	"strconv"
 	"sync"
+	"time"
 
 	"github.com/pkg/errors"
 	"github.com/rclone/rclone/cmd"
@@ -503,6 +504,11 @@ func (f *FileInfo) Group() string {
 	return g.Name
 }
 
+// ModTime returns the time in UTC
+func (f *FileInfo) ModTime() time.Time {
+	return f.FileInfo.ModTime().UTC()
+}
+
 func closeIO(path string, c io.Closer) {
 	err := c.Close()
 	if err != nil {