build: filter out CTags setting from flag parse test

The CTags default value depends on $PATH, and whether ctags is
installed.

Change-Id: I3e6fd9d5e69ddbe37120eb0cab9c3f1208184dac
diff --git a/build/builder_test.go b/build/builder_test.go
index 60c8be4..133b39a 100644
--- a/build/builder_test.go
+++ b/build/builder_test.go
@@ -36,6 +36,8 @@
 
 	for _, c := range cases {
 		c.want.SetDefaults()
+		// depends on $PATH setting.
+		c.want.CTags = ""
 
 		got := Options{}
 		fs := flag.NewFlagSet("", flag.ContinueOnError)