ci: cron schedule for workflows, merge_group support.

This commit does two things:

* In preparation for one day enabling merge queue on this repo, it adds
  `merge_group` as a trigger for the CI tasks.
* It adds a `schedule` trigger that will run the CI tasks weekly, to
  detect bitrot sooner than we might otherwise.

Author: cpu

.github/workflows/test.yaml

@@ -3,7 +3,12 @@ name: rustls-ffi
 permissions:
   contents: read
 
-on: [push, pull_request]
+on:
+  push:
+  pull_request:
+  merge_group:
+  schedule:
+    - cron: '15 12 * * 3'
 
 jobs:
   build: