Refactor factory.Executable() to be a method rather than a func

This way, factory can satisfy an interface that requires `Executable()`.

pkg/cmd/auth/login/login_test.go

@@ -147,8 +147,7 @@ func Test_NewCmdLogin(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			io, stdin, _, _ := iostreams.Test()
 			f := &cmdutil.Factory{
-				IOStreams:  io,
-				Executable: func() string { return "/path/to/gh" },
+				IOStreams: io,
 			}
 
 			io.SetStdoutTTY(true)