google: check additional AWS variable

AWS_DEFAULT_REGION should have been checked as a backup to AWS_REGION but wasn't.  Also removed a redundant print statement in a test case.

Change-Id: Ia6e13eb20f509110a81e3071228283c43a1e9283
GitHub-Last-Rev: 1a10bcc0791f862983c3e3ae36f0cb73e29db267
GitHub-Pull-Request: golang/oauth2#486
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/302789
Reviewed-by: Cody Oss <codyoss@google.com>
Trust: Cody Oss <codyoss@google.com>
Trust: Tyler Bui-Palsulich <tbp@google.com>

google/internal/externalaccount/urlcredsource_test.go

@@ -7,7 +7,6 @@ package externalaccount
 import (
 	"context"
 	"encoding/json"
-	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -20,7 +19,6 @@ func TestRetrieveURLSubjectToken_Text(t *testing.T) {
 		if r.Method != "GET" {
 			t.Errorf("Unexpected request method, %v is found", r.Method)
 		}
-		fmt.Println(r.Header)
 		if r.Header.Get("Metadata") != "True" {
 			t.Errorf("Metadata header not properly included.")
 		}