chore: refactor to use const instead of var (#1731)

tools/goctl/util/head.go

@@ -1,10 +1,12 @@
 package util
 
-// DoNotEditHead added to the beginning of a file to prompt the user not to edit
-var DoNotEditHead = "// Code generated by goctl. DO NOT EDIT!"
+const (
+	// DoNotEditHead added to the beginning of a file to prompt the user not to edit
+	DoNotEditHead = "// Code generated by goctl. DO NOT EDIT!"
 
-var headTemplate = `// Code generated by goctl. DO NOT EDIT!
+	headTemplate = DoNotEditHead + `
 // Source: {{.source}}`
+)
 
 // GetHead returns a code head string with source filename
 func GetHead(source string) string {