Add schedule trigger event to integration test workflow

The integration tests use the latest release of Arduino Lint. This means that changes in Arduino Lint can cause an
integration test failure independent from any change in this repository. Without a schedule trigger, this might only be
noticed as an irrelevant CI failure for the next PR that is submitted, which can cause confusion and inconvenience to the
contributor.

A schedule trigger allows the breakage to be identified and resolved early.

Author: per1234

.github/workflows/test-integration.yml

@@ -22,6 +22,9 @@ on:
       - "poetry.lock"
       - "pyproject.toml"
       - "test/**"
+  schedule:
+    # Run daily at 8 AM UTC to catch breakage resulting from changes to Arduino Lint.
+    - cron: "0 8 * * *"
   workflow_dispatch:
     inputs:
       arduino-lint-ref: