Run debug CI nightly instead of on every PR

Backport of duckdb#1121 — debug CI
runs are slow (>2h), so move to a nightly schedule plus manual dispatch
instead of running on every push/PR.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: bill-ph

.github/workflows/Debug.yml

@@ -1,5 +1,8 @@
 name: Debug Mode Tests
-on: [push, pull_request,repository_dispatch]
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: '0 2 * * *'
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}-${{ github.head_ref || '' }}-${{ github.base_ref || '' }}-${{ github.ref != 'refs/heads/main' || github.sha }}
   cancel-in-progress: true