Change experiments config fetch log to be a verbose log

This log is not super useful to be honest, so avoid printing it to
stdout of the build.

Bug: b/236709761
Change-Id: Ic96130a285160b386df3afc269dd5ef97c7965bf
diff --git a/ui/build/config.go b/ui/build/config.go
index 5765f21..59b01b3 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -211,7 +211,7 @@
 	}
 
 	if err := fetchEnvConfig(ctx, config, bc); err != nil {
-		fmt.Fprintf(os.Stderr, "Failed to fetch config file: %v\n", err)
+		ctx.Verbosef("Failed to fetch config file: %v\n", err)
 	}
 
 	configDirs := []string{