Schedule a Go package refresh when a tag is pushed

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Followup, to reduce the scope of !29642 (closed). To improve the user experience, reduce the work done by Packages::Go::RefreshPackagesService, and reduce the frequency with which the user schedules a refresh:

When the user pushes a tag, call Packages::Go::RefreshPackagesService#execute_async for that tag.

/cc @trizzi @10io