[ci] Run 6.30 nighly only every other night

Author: dpiparo

.github/workflows/root-630.yml

@@ -3,7 +3,7 @@ name: 'ROOT 6.30'
 
 on:
   schedule:
-    - cron: '30 1 * * *'
+    - cron: '30 1 1-31/2 * *'
 
   workflow_dispatch:
     inputs: