Add a CI job that checks whether a Git tag matching @core/sync-service has appeared to trigger a build on Buildkite

[alco]

Our current Buildkite setup relies on GitHub sending a webhook for each new tag. But there's a known problem where GitHub does not send a webhook for commits that have 4+ tags pushed alongside the commit at once.

We can fix this by triggering a new build on Buildkite manually after we've identified a new tag matching the desired pattern has been pushed, regardless of which other tags have been pushed together with it.