Run the build on 'main' periodically to keep the caches alive (#50)

Notes from Github:

* In a public repository, scheduled workflows are automatically disabled when no repository activity has occurred in 60 days. For information on re-enabling a disabled workflow, see "Disabling and enabling a workflow."

* When the last user to commit to the cron schedule of a workflow is removed from the organization, the scheduled workflow will be disabled. If a user with write permissions to the repository makes a commit that changes the cron schedule, the scheduled workflow will be reactivated. Note that, in this situation, the workflow is not reactivated by any change to the workflow file; you must alter the cron value and commit this change.

.github/workflows/build.yml

@@ -10,12 +10,15 @@ on:  # yamllint disable-line rule:truthy
       - "**"
     tags:
       - "v*"
+  schedule:
+    - cron: "31 18 * * sat"  # Weekly, Saturday at 18:31 UTC
 
 env:
   UNITY_VERSION: "2019.4.25f1"
 jobs:
   configuration:
     if: |
+      (github.event_name == 'schedule') ||
       (github.event_name == 'pull_request') ||
       (
         github.event_name == 'push' &&