fix: run build in screen session to survive SSH disconnects

Long builds (NVIDIA driver download+compile) would abort on SSH timeout.
Now build runs in a detached screen session on the VM, run-builder.sh
streams the log and waits for completion safely.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/setup-builder.sh

@@ -37,7 +37,8 @@ apk add \
     wget \
     curl \
     tar \
-    xz
+    xz \
+    screen
 
 # --- audit runtime packages (verify they exist in Alpine repos) ---
 echo ""