GitHub: Disable VirtualBox builds

I have not used VirtualBox in years.  For some unknown reason, the
builds are timing out.  Without a console output, debugging is hard.

Disable the jobs on VirtualBox until a solution is found.

.github/workflows/packer.yml

@@ -49,6 +49,7 @@ jobs:
           ntpdate_hosts: time.apple.com
 
   virtualbox:
+    if: false
     runs-on: macos-13
     name: VirtualBox Builder
     needs: validate