dashboard: fix ssh username for ppc64 builders
dashboard: fix ssh username for ppc64 builders

Updates golang/go#34739

Change-Id: I9e9bcf66a4e3939a68178b344672a1bab2698bb4
Reviewed-on: https://go-review.googlesource.com/c/build/+/200377
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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