ICU-22711 Set EnvTest frequency to nightly in March/April/Sept/Oct

.github/workflows/icu_envtest.yml

@@ -14,6 +14,9 @@ on:
  schedule:
  # This cron schedule is set to run 10:23 UTC every SAT
  - cron: '23 10 * * SAT'
+ # In addition, during the major release cycle (in March/April, September/October),
+ # this cron schedule is set to run 10:21 UTC every Workday and Sunday.
+ - cron: '21 10 * MAR,APR,SEP,OCT MON-FRI,SUN'
 
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