Not working when using a pointer to a custom type

Nobody has claimed this yet.

I have a custom ISBN type which is defined `type ISBN [13]byte`

I am testing that I can use the Scan method with it, both as a value type and a pointer type. The latter gives me the error `can't scan into dest[1]: unable to assign to *[13]uint8`

I here is a snippit of my test code

```
type testcase struct {
ID int
Isbn *ISBN // nullable
Title *[13]byte // nullable
}

assert.NilError(t, db.QueryRow(context.Background(), `SELECT id, isbn, title FROM foo WHERE id = 1`).Scan(&tc.ID, &tc.Isbn, &tc.Title))
```

For some reason the Title one works however I can't get my custom type to work.

Here is what my `Scan` method looks like:

```
func (isbn *ISBN) Scan(v any) (err error) {
switch u := v.(type) {
case []byte: // sqlite3
*isbn, err = ParseBytes(u)
case string:
*isbn, err = Parse(u)
default:
return invalidTypeError{reflect.TypeOf(v)}
}
return err
}
```

It doesn't even seem to get called so I am unsure what else I need in order to get it to work

No repository file or test is named. Start by reproducing the Scan call with the custom ISBN and pointer destinations, then trace the relevant pgx scanning path to determine why ISBN.Scan is not reached; done means the pointer case has a verified, documented outcome or a regression test covering the expected behavior.

Written by the indexing model from the issue text.