Prevent builtin extensions from being updated

Updating builtin extensions from the marketplace prevents us from patching them
(for example out GitHub authentication patches).

Index: code-server/lib/vscode/src/vs/workbench/contrib/extensions/browser/extensionsWorkbenchService.ts
===================================================================
--- code-server.orig/lib/vscode/src/vs/workbench/contrib/extensions/browser/extensionsWorkbenchService.ts
+++ code-server/lib/vscode/src/vs/workbench/contrib/extensions/browser/extensionsWorkbenchService.ts
@@ -283,6 +283,10 @@ export class Extension implements IExten
 			if (this.type === ExtensionType.System && this.productService.quality === 'stable') {
 				return false;
 			}
+			// Do not update builtin extensions.
+			if (this.type !== ExtensionType.User) {
+				return false;
+			}
 			if (!this.local.preRelease && this.gallery.properties.isPreReleaseVersion) {
 				return false;
 			}