tools: fix show_manual_release_cmd.vsh, use .strftime(%V) for calcula…

…ting the proper default week number

.github/workflows/show_manual_release_cmd.vsh

@@ -1,11 +1,10 @@
 import os
 import time
-import math
 import term
 
 now := time.now()
 year := now.year
-week := int(math.ceil(f64(now.year_day()) / 7.0))
+week := now.strftime('%V')
 
 mut remote_name := 'origin'
 mut release_tag := 'weekly.${year:04}.${week:02}'