Let a completed successful tag-git workflow run trigger the corresponding git-artifacts runs

[dscho]

To finish porting the current Azure Pipeline to GitHub Actions, which we used as an opportunity to split off the tagging into its own GitHub workflow (tag-git, we need to teach the /git-artifacts command to trigger that tag-git workflow, and upon successful completion trigger the git-artifacts workflow runs (one per architecture).

This requires the GitForWindowsHelper to pay attention not only to queued and completed Windows/ARM64 workflow jobs but also to completed workflow runs.

And to make things more transparent, we need to update the comment that reported that the workflow runs were started, much like #9 suggests.

[dscho]

Done, via 01c51fa