chore(builder): add hidden sign-files command for testing purposes

Useful when needing to sign singular files with same ease of signing a
Emacs.app bundle.

pkg/cli/cli.go

@@ -46,6 +46,7 @@ func New(version, commit, date string) *CLI {
 			Commands: []*cli2.Command{
 				planCmd(),
 				signCmd(),
+				signFilesCmd(),
 				notarizeCmd(),
 				packageCmd(),
 				releaseCmd(),