gha: ci: schedule expects a list, not a map

And because of that we need to declare '- cron', instead of 'cron'.

Signed-off-by: Fabiano Fidêncio <fabiano.fidencio@intel.com>

.github/workflows/ci-nightly.yaml

@@ -1,7 +1,7 @@
 name: Kata Containers Nightly CI
 on:
   schedule:
-    cron: '0 0 * * *'
+    - cron: '0 0 * * *'
 
 env:
   COMMIT_HASH: ${GITHUB_REF}