Update Github CI status indicator when invoking `@ofborg eval`

[pbsds]

Sometimes eval fail due to a socket error or something, and posting @ofborg eval will fix it. The problem is that the CI badge remains a red X until the eval has been completed, which may take a few hours depending on the queue. This poor feedback has led to many instead opting to rebase-force-push the PR rather than invoke @ofborg eval, simply because they think @ofborg eval doesn't work. This in turn makes viewing the diff between force-pushes unviable.

It would be great if the CI status indicators immediately turn yellow when invoking @ofborg eval, like they already do for @ofborg build xyz