fix: ensure Arborist constructor gets passed around everywhere for pacote (#5634)

Author: nlf

lib/npm.js

@@ -1,3 +1,4 @@
+const Arborist = require('@npmcli/arborist')
 const EventEmitter = require('events')
 const { resolve, dirname, join } = require('path')
 const Config = require('@npmcli/config')
@@ -310,6 +311,9 @@ class Npm extends EventEmitter {
 
   get flatOptions () {
     const { flat } = this.config
+    // the Arborist constructor is used almost everywhere we call pacote, it's easiest
+    // to attach it to flatOptions so it goes everywhere without having to touch every call
+    flat.Arborist = Arborist
     if (this.command) {
       flat.npmCommand = this.command
     }