goauth2/compute/serviceaccount: fix required header for metadata v1

https://developers.google.com/compute/docs/metadata#transitioning
says v1 requires "Metadata-Flavor: Google" instead of
"X-Google-Metadata-Request: True".

R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/124770044
1 file changed