CI: Scheduling daily builds (#426)

Author: David Teller

.github/workflows/mjolnir.yml

@@ -5,7 +5,8 @@ on:
     branches: [ main ]
   pull_request:
     branches: [ main ]
-
+  schedule:
+    - cron: '20 20 * * *'
 env:
   CARGO_TERM_COLOR: always