Set up debian image with zink/lavapipe

Set env var to run GLX app with zink backed by lavapipe

Bug: 393548134
Test: . enable_dispaly, and then `glxinfo -B` shows zink, and `glxgears`
works well(need to install mesa-utils to test)

Change-Id: Iaf12855a3ff67acdb96de972f67012adf3d8d9f6
diff --git a/build/debian/vm_config.json.aarch64 b/build/debian/vm_config.json.aarch64
index 96254f8..463583f 100644
--- a/build/debian/vm_config.json.aarch64
+++ b/build/debian/vm_config.json.aarch64
@@ -35,5 +35,8 @@
     "console_out": true,
     "console_input_device": "ttyS0",
     "network": true,
-    "auto_memory_balloon": true
+    "auto_memory_balloon": true,
+    "gpu": {
+        "backend": "2d"
+    }
 }