addon/*: move tabWidth definition to local scope.

This seemingly introduces duplication, but IMO this constant is a style implementation detail that doesn't make much sense to share between different places.
Column-based pretty-printing looks depend on the exact data being shown. If we think that it's the same data being printed in both places, then we should extract & share a bigger chunk of code than just one const.

cmd/wksctl/addon/list/list.go

@@ -20,6 +20,7 @@ func init() {
 }
 
 func addonListRun(cmd *cobra.Command, args []string) {
+	const tabWidth = 4
 	w := tabwriter.NewWriter(os.Stdout, 0, 0, tabWidth, ' ', 0)
 
 	addons := addons.List()

cmd/wksctl/addon/show/show.go

@@ -35,6 +35,7 @@ func addonShowRun(cmd *cobra.Command, args []string) {
 		log.Fatal(err)
 	}
 
+	const tabWidth = 4
 	w := tabwriter.NewWriter(os.Stdout, 0, 0, tabWidth, ' ', 0)
 
 	fmt.Fprintf(w, "Name\t%s\n", addon.Name)

cmd/wksctl/const.go

-package main
-
-const (
-	// tabWidth is the number of spaces between tabWriter columns.
-	tabWidth = 4
-)