sync-mailing-list-mirror: use more elegant cron definition

Rather than hard-coding the exact minutes at which the workflow should
run, just specify that it should run every five minutes, and leave it to
GitHub to figure out the specifics.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

.github/workflows/sync-mailing-list-mirror.yml

@@ -3,7 +3,7 @@ name: sync-mailing-list-mirror
 on:
   workflow_dispatch:
   schedule:
-    - cron:  "2,7,12,17,22,27,32,37,42,47,52,57 * * * *"
+    - cron:  "*/5 * * * *"
 
 env:
   LORE_EPOCH: 1 # also adjust SOURCE_REPOSITORY