panic on scanning to slice of custom types

**Describe the bug**
Library panics on scanning to []CustomType

**To Reproduce**
```go
package main

import (
"context"

"github.com/jackc/pgx/v4/pgxpool"
)

type Int64 int64
func main() {
conn,err := pgxpool.Connect(context.Background(), "postgres://user:qwert1@localhost/tests?port=5432&application_name=subbotin_pidor")
if err !=nil {
panic(err)
}
_, err = conn.Exec(context.Background(), "CREATE TABLE IF NOT EXISTS test (id serial PRIMARY KEY, ids bigint[])")
if err != nil {
panic(err)
}
_, err = conn.Exec(context.Background(), "INSERT INTO test (ids) VALUES ($1)", []Int64{1,2,3})
if err != nil {
panic(err)
}
var ids []Int64
err = conn.QueryRow(context.Background(), "SELECT ids FROM test WHERE id = 1").Scan(&ids)
if err != nil {
panic(err)
}
print(ids)
}

```

**Expected behavior**
Printing `1, 2, 3`

**Actual behavior**
```
panic: reflect.Value.Convert: value of type *[]main.Int64 cannot be converted to type *[]int64

goroutine 1 [running]:
reflect.Value.Convert({0x727320?, 0xc0002020a8?, 0x3?}, {0x81cde8, 0x7276e0})
/home/user/sdk/go1.22.0/src/reflect/value.go:3368 +0x12c
github.com/jackc/pgtype.toInterface({0x727320?, 0xc0002020a8?, 0x6?}, {0x81cde8?, 0x7276e0?})
/home/user/go/pkg/mod/github.com/jackc/pgtype@v1.12.0/convert.go:397 +0x46
github.com/jackc/pgtype.GetAssignToDstType({0x727320?, 0xc0002020a8?})
/home/user/go/pkg/mod/github.com/jackc/pgtype@v1.12.0/convert.go:431 +0x2a6
github.com/jackc/pgtype.(*Int8Array).AssignTo(0xc000072e80, {0x727320, 0xc0002020a8})
/home/user/go/pkg/mod/github.com/jackc/pgtype@v1.12.0/int8_array.go:613 +0x885
github.com/jackc/pgtype.(*scanPlanDataTypeAssignTo).Scan(0xc00007e5a0, 0xc00010abd0, 0x3f8, 0x1, {0xc0001b2336, 0x38, 0x38}, {0x727320, 0xc0002020a8})
/home/user/go/pkg/mod/github.com/jackc/pgtype@v1.12.0/pgtype.go:590 +0x130
github.com/jackc/pgx/v4.(*connRows).Scan(0xc00021a000, {0xc000210020?, 0x1, 0xc000210020?})
/home/user/go/pkg/mod/github.com/jackc/pgx/v4@v4.17.2/rows.go:225 +0x3fd
github.com/jackc/pgx/v4.(*connRow).Scan(0xc00021a000, {0xc000210020, 0x1, 0x1})
/home/user/go/pkg/mod/github.com/jackc/pgx/v4@v4.17.2/rows.go:88 +0x8e
github.com/jackc/pgx/v4/pgxpool.(*poolRow).Scan(0xc0000e69e0, {0xc000210020?, 0xa87f40?, 0x796afb?})
/home/user/go/pkg/mod/github.com/jackc/pgx/v4@v4.17.2/pgxpool/rows.go:100 +0x31
main.main()
```

**Version**
- Go: v2.21
- PostgreSQL: 14
- pgx: github.com/jackc/pgx/v4 v4.17.2

Reproduce the example against PostgreSQL, then start with the stack-trace locations in pgtype/convert.go and pgtype/int8_array.go. Trace the assignment path for []Int64 and add a regression test covering scanning a bigint array into a slice of a custom Go type; done means it returns [1 2 3] without panicking.

Written by the indexing model from the issue text.