Run tests every day to assure compatibility (#251)

Because I run `composer update` in PHP workflow, it should help detect things like #249 earlier.

.github/workflows/php.yml

@@ -1,6 +1,11 @@
 name: PHP Tests
 
-on: [push, pull_request]
+on:
+    push:
+    pull_request:
+    workflow_dispatch:
+    schedule:
+        - cron: '24 20 * * *'
 
 jobs:
   lint-phpcs-phpstan: