[Bug] Adopt using platform specifier for built-in extensions

[rrmistry]

Does this issue occur when all extensions are disabled?: Yes

• VS Code Version: 1.96.4
• OS Version: Web with linux-x64 host

Steps to Reproduce:

1. Add a built-in extension to product.json (see example below for ms-python.debugpy):

   {
       // ...
       "extensionsGallery": {
           "serviceUrl": "https://open-vsx.org/vscode/gallery",
           "itemUrl": "https://open-vsx.org/vscode/item",
           "resourceUrlTemplate": "https://open-vsx.org/api/{publisher}/{name}/{targetPlatform}/{version}/file/{path}"
       },
       "linkProtectionTrustedDomains": [
           "https://open-vsx.org"
       ],
       "builtInExtensions": [
           // ...
           {
               "name": "ms-python.debugpy",
               "version": "latest",
               "platforms": [
                   "linux-x64",
                   "linux-arm64"
               ],
               "repo": "https://github.com/microsoft/vscode-python-debugger",
               "metadata": {
                   "publisherId": {
                       "publisherName": "ms-vscode",
                       "displayName": "Microsoft",
                       "flags": "verified"
                   },
                   "publisherDisplayName": "Microsoft"
               }
           }
       ]
   }

2. Build the extensions list with command:

   npm run gulp compile-extensions-build

[rrmistry]

Related: #180525 / PR: #182072