ci: create /var/tmp as buildah does not allow overriding the tmp dir

flake.nix

@@ -127,6 +127,7 @@
                   mkdir -p /etc/containers &&
                     echo '{"default":[{"type":"insecureAcceptAnything"}]}' > /etc/containers/policy.json
                 ''
+                ''mkdir -p /var/tmp''
               ];
               script = [
                 ''