Run mac jobs on labeled hosts

This is needed to support an upcoming management script change.
I've already updated all the workers to be sensitive
to this label.

Ref:
https://github.com/cirruslabs/cirrus-cli/blob/master/PERSISTENT-WORKERS.md#reserved-labels

Signed-off-by: Chris Evich <cevich@redhat.com>

.cirrus.yml

@@ -454,6 +454,7 @@ osx_alt_build_task:
         labels:
             os: darwin
             arch: arm64
+            purpose: prod
     env: &mac_env
         CIRRUS_SHELL: "/bin/bash"  # sh is the default
         CIRRUS_WORKING_DIR: "$HOME/ci/task-${CIRRUS_TASK_ID}"  # Isolation: $HOME will be set to "ci" dir.