CI: Build OD every month

Prevent the build artifacts from getting removed after 3 months, by
generating new ones every month.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

Author: pcercuei

.github/workflows/main.yml

@@ -8,6 +8,8 @@ on:
   pull_request:
     branches:
       - opendingux
+  schedule:
+    - cron: '* * 1 * *'
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}