spread.yaml: set default nested channel to beta

Channel for kernel cannot be edge as that one is not signed with
Canonical keys anymore, so we default the image to beta.

spread.yaml

@@ -85,7 +85,7 @@ environment:
     # Directory where the nested images and test assets are stored
     NESTED_WORK_DIR: '$(HOST: echo "${NESTED_WORK_DIR:-/tmp/work-dir}")'
     # Channel used to create the nested vm
-    NESTED_CORE_CHANNEL: '$(HOST: echo "${NESTED_CORE_CHANNEL:-edge}")'
+    NESTED_CORE_CHANNEL: '$(HOST: echo "${NESTED_CORE_CHANNEL:-beta}")'
     # Use cloud init to make initial system configuration instead of user assertion
     NESTED_CORE_REFRESH_CHANNEL: '$(HOST: echo "${NESTED_CORE_REFRESH_CHANNEL:-edge}")'
     # Use cloud init to make initial system configuration instead of user assertion