Add scheduled CI build to prevent bitrot

Author: AMDmi3

.github/workflows/ci.yml

@@ -1,5 +1,9 @@
 name: CI
-on: [ push, pull_request ]
+on:
+  push:
+  pull_request:
+  schedule:
+    - cron: "0 0 1 * *"
 jobs:
   build:
     runs-on: ubuntu-22.04