fix(nsis): Cloning packager.config to prevent override

This will fix the last issue of #1340 when targets artifact's names are changed.

Close #1340

packages/electron-builder/src/targets/nsis.ts

@@ -88,7 +88,7 @@ export class NsisTarget extends Target {
 
     this.packageHelper.refCount++
 
-    let options = this.packager.config.nsis || Object.create(null)
+    let options = Object.assign({}, this.packager.config.nsis) || Object.create(null)
     if (targetName !== "nsis") {
       options = Object.assign(options, (<any>this.packager.config)[targetName === "nsis-web" ? "nsisWeb" : targetName])
     }
@@ -579,4 +579,4 @@ function computeCustomMessageTranslations(messages: any): Array<string> {
 
 function createMacro(name: string, lines: Array<string>) {
   return `\n!macro ${name}\n  ${lines.join("\n  ")}\n!macroend\n`
-}
+}