github actions: runs the integration test nightly

The secret in github actions won't be passed to
actions triggered by pull request. What's more,
the time consumed is unacceptable. Therefore,
we change integration test to nightly test.

Signed-off-by: Qi Wang <wangqi@linux.alibaba.com>

.github/workflows/it.yml

@@ -1,13 +1,9 @@
 name: Nydus Ingegration Test
 
 on:
-  push:
-    branches: ["*"]
-  pull_request:
-    branches: ["*"]
   schedule:
-    # Run daily sanity check at 23:08 clock UTC
-    - cron: "8 23 * * *"
+    # Do conversion every day at 00:03 clock UTC
+    - cron: "3 0 * * *"
   workflow_dispatch:
 
 jobs: