Adds check for aws values in environment variables before the metadata server is called to prevent unnecessary off box calls. See https://github.com/googleapis/google-auth-library-java/pull/1100 for same change in java library.
Change-Id: Ie86a899be88c38d3fcbbe377f9bf30a7a66530c0
GitHub-Last-Rev: bcab69572c
GitHub-Pull-Request: golang/oauth2#612
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/453715
Reviewed-by: Leo Siracusa <leosiracusa@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cody Oss <codyoss@google.com>
Auto-Submit: Cody Oss <codyoss@google.com>
Reviewed-by: Cody Oss <codyoss@google.com>