build: name temp files as *.tmp

Change-Id: I83791dc61510c9ef4b9635e7366afb2b7540941b
diff --git a/build/builder.go b/build/builder.go
index 6c35e50..d1fc5f3 100644
--- a/build/builder.go
+++ b/build/builder.go
@@ -533,7 +533,7 @@
 		return nil, err
 	}
 
-	f, err := ioutil.TempFile(dir, filepath.Base(fn))
+	f, err := ioutil.TempFile(dir, filepath.Base(fn) + ".*.tmp")
 	if err != nil {
 		return nil, err
 	}