Speed up build_test

Stop writing out ninja files, as they become very large, especially when
multiplied by the number of defined products.

Test: treehugger
Change-Id: Id9529d14040acb72a0188e58b5db2911f142071e
diff --git a/ui/build/kati.go b/ui/build/kati.go
index 56e9a88..d2e9fe9 100644
--- a/ui/build/kati.go
+++ b/ui/build/kati.go
@@ -81,6 +81,10 @@
 		"--kati_stats",
 	}, args...)
 
+	if config.Environment().IsEnvTrue("EMPTY_NINJA_FILE") {
+		args = append(args, "--empty_ninja_file")
+	}
+
 	cmd := Command(ctx, config, "ckati", executable, args...)
 	cmd.Sandbox = katiSandbox
 	pipe, err := cmd.StdoutPipe()