build all images every day to get the latest libraries

Version incompabilities between library versions in the build image and
the runtime image can otherwise cause segfaults and other problems.

See: cloudamqp/amqproxy#178

.github/workflows/ci.yml

@@ -8,6 +8,8 @@ on:
     paths-ignore:
       - README.md
       - .github/workflows/master.yml
+  schedule:
+    - cron: '45 4 * * *'
 
 env:
   crystal_version: 1.13.3