Adding explicit references to subsystemregistration and arsubsystems packages for now.

Packages/manifest.json

     "com.unity.collab-proxy": "1.2.16",
     "com.unity.package-manager-ui": "2.0.7",
     "com.unity.purchasing": "2.0.6",
+    "com.unity.subsystemregistration": "1.0.5",
+    "com.unity.xr.arsubsystems": "2.1.0-preview.1",
     "com.unity.modules.ai": "1.0.0",
     "com.unity.modules.animation": "1.0.0",
     "com.unity.modules.assetbundle": "1.0.0",