workflows/sync-shared-config: add workflow_dispatch trigger

This allows us to run this workflow without having to wait for up to 3
hours. Needed to unblock #206
where CI fails due to `.ruby-version` not being in sync with `brew`'s
after a new Portable Ruby release.

Author: ZhongRuoyu

.github/workflows/sync-shared-config.yml

@@ -7,6 +7,7 @@ on:
   pull_request:
   schedule:
     - cron: "0 */3 * * *" # Every 3 hours
+  workflow_dispatch:
 
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