cmd/coordinator: fix staging instance

- Make URLs point to correct external IP
- Disable windows-amd64-2008 builder type that doesn't exist in
  the staging farm

Updates golang/go#18817

Change-Id: Id64a63694f90e70c4fd78f9d1433ed5031822111
Reviewed-on: https://go-review.googlesource.com/42850
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed