Fix macOS universal build for electron-builder 26.4.0

electron-builder 26.4.0 includes a stricter version of @electron/universal
that validates Mach-O binaries during universal app creation. The bufferutil
package installs prebuilt binaries for all architectures (darwin-x64 and
darwin-arm64), causing the universal merger to detect identical files in
both builds and fail.

Add x64ArchFiles pattern to allow these architecture-specific prebuilds
to pass validation without attempting to merge them with lipo.

apps/desktop/electron-builder.json

@@ -72,6 +72,7 @@
     },
     "provisioningProfile": "bitwarden_desktop_developer_id.provisionprofile",
     "singleArchFiles": "node_modules/@bitwarden/desktop-napi/desktop_napi.darwin-*.node",
+    "x64ArchFiles": "**/bufferutil/**/darwin-*/*.node",
     "extraFiles": [
       {
         "from": "desktop_native/dist/desktop_proxy.${platform}-${arch}",