Include distro-specific vars

We need ca_dir to copy the certs in, but when we run in multi-node
cases the use-buildset-registry invocation is in a separate place
so we dont' leak the var in.

This will get deleted in the next patch, but that patch is not
working and we need this to unbreak the multi-arch functionality
for nodepool.

Change-Id: I4f92f0415fb471f304fdd0a1e576812c8d67ab24

roles/build-docker-image/tasks/setup-buildx.yaml

@@ -1,3 +1,14 @@
+- name: Include OS-specific variables
+  include_vars: "{{ zj_distro_os }}"
+  with_first_found:
+    - "{{ ansible_distribution }}-{{ ansible_distribution_major_version }}.yaml"
+    - "{{ ansible_distribution }}.{{ ansible_architecture }}.yaml"
+    - "{{ ansible_distribution }}.yaml"
+    - "{{ ansible_os_family }}.yaml"
+    - "default.yaml"
+  loop_control:
+    loop_var: zj_distro_os
+
 - name: Make tempfile for buildkit.toml
   tempfile:
     state: file