x/perf/cmd/benchsave uses Google's out-of-band authentication flow which has been deprecated, so benchsave can no longer authenticate, or thus upload benchmark results.

I think the best path forward here is to migrate perfdata.golang.org to x/build's GKE cluster, at which point we can put perfdata.golang.org/upload behind IAP like other x/build authenticated services. At that point, benchsave can reuse x/build/internal/iapclient, the same authentication client used by gomote.

This is not the quickest path, but this migration is something we've put off for a while and doing the long path won't be leaving tech debt behind.

cc @mknyszek @aclements