[amber] use the configured transport with oauth2

Test: CQ
Change-Id: I473fd4946aa3ffa4e03c4f9f531117109c821753
diff --git a/go/src/amber/source/source.go b/go/src/amber/source/source.go
index c2deb1e..4939c69 100644
--- a/go/src/amber/source/source.go
+++ b/go/src/amber/source/source.go
@@ -372,11 +372,13 @@
 	ctx := context.WithValue(context.Background(), oauth2.HTTPClient, httpClient)
 
 	timeout := httpClient.Timeout
+	transport := httpClient.Transport
 
 	tokenSource = cfg.TokenSource(ctx, token)
 	httpClient = oauth2.NewClient(ctx, tokenSource)
 
 	httpClient.Timeout = timeout
+	httpClient.Transport = transport
 
 	return httpClient, tokenSource
 }