fix(packager): fix custom afterCopy, afterPrune not being included

As per MDN -->
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/concat the
concat method does not change existing arrays but returns a new one.  We can use the spread operator
and push to achieve what we want

Author: MarshallOfSound

src/api/package.js

@@ -115,13 +115,13 @@ export default async (providedOptions = {}) => {
     done();
   });
 
-  afterCopyHooks.concat(resolveHooks(forgeConfig.electronPackagerConfig.afterCopy, dir));
+  afterCopyHooks.push(...resolveHooks(forgeConfig.electronPackagerConfig.afterCopy, dir));
 
   const afterPruneHooks = [];
 
   if (pruneEnabled) {
     afterPruneHooks.push(rebuildHookFn);
-    afterPruneHooks.concat(resolveHooks(forgeConfig.electronPackagerConfig.afterPrune, dir));
+    afterPruneHooks.push(...resolveHooks(forgeConfig.electronPackagerConfig.afterPrune, dir));
   }
 
   const packageOpts = Object.assign({