auto-close failed dependabot PRs

[dannysauer]

I think we should probably go ahead and close dependabot PRs which don't build. It's not the end of the world if they hang around and get rebased until they actually work, but dependabot has an upper limit on how many it opens at one time. So, to avoid inadvertently going over that limit and missing valid updates, we probably need to clean PRs up.

[dannysauer]

It might be possible to look for "resolution Impossible" in the build output, possibly adding a label if that happens and clearing it when the job restarts.