cmd/gopherbot: ignore GitHub PRs when updating issues
cmd/gopherbot: ignore GitHub PRs when updating issues

Otherwise you get something like
https://github.com/golang/go/pull/23752#issuecomment-364288277

Update golang/go#18517

Change-Id: I5298f1e34738a4f6faa8b829546bf880a237043d
Reviewed-on: https://go-review.googlesource.com/93015
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build