android-emulator: api36-v3 — avdmanager must run from inside the SDK root

v2's marker fix proved the install completes, but avdmanager still saw
no system images: it IGNORES ANDROID_SDK_ROOT (and has no --sdk_root),
deriving the SDK root from its own toolsdir — /opt/android in our image,
while packages live on the PVC at /sdk. v3 seeds cmdline-tools into
/sdk/cmdline-tools/latest once and runs avdmanager from there, so it
resolves the PVC as the SDK root.

stacks/android-emulator/variables.tf

@@ -5,6 +5,6 @@ variable "tls_secret_name" {
 
 variable "image_tag" {
   type        = string
-  default     = "api36-v2"
+  default     = "api36-v3"
   description = "android-emulator image tag at forgejo.viktorbarzin.me/viktor/android-emulator. Built + pushed manually from stacks/android-emulator/docker/ (see README.md) — bump this when the image is rebuilt."
 }