Fragment interpolation breaks with nested fragments

[taig]

import skunk.*
import skunk.codec.all.*
import skunk.syntax.all.*

def updateBy[A](where: Fragment[A]): Command[(String, A)] =
  sql"""UPDATE foo SET bar = $text WHERE $where;""".command

Found:    skunk.Command[String *: Nothing *: Nothing]
Required: skunk.Command[(String, A)]

https://scastie.scala-lang.org/KeXx8O03QROMVXV9Cx8ihQ

[mpilquist]

Fixed by #991