Cirrus: Trigger podman-machine task by label

Instead of requiring developers to search for a magic button, make the
task trigger at the time a special PR label is added.  Update comments
accordingly.

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

.cirrus.yml

@@ -588,11 +588,14 @@ rootless_integration_test_task:
 podman_machine_task:
     name: *std_name_fmt
     alias: podman_machine
+    # Required_pr_labels does not apply to non-PRs.
+    # Do not run on tags, branches, [CI:BUILD], or [CI:DOCS].
     only_if: *not_tag_branch_build_docs
-    # Manually-triggered task: This is "expensive" to run.
+    # This task costs about $4 per attempt to execute.
+    # Only run it if a magic PR label is present.
     # DO NOT ADD THIS TASK AS DEPENDENCY FOR `success_task`
-    # it will cause 'success' to block.
-    trigger_type: manual
+    # it will cause an infinate-block / never completing build.
+    required_pr_labels: test_podman_machine
     depends_on:
         - build
         - local_integration_test
@@ -879,8 +882,8 @@ success_task:
         - remote_integration_test
         - container_integration_test
         - rootless_integration_test
-        # Manually triggered task. If made automatic, remove bypass
-        # in contrib/cirrus/cirrus_yaml_test.py for this task.
+        # Label triggered task. If made automatic, remove line below
+        # AND bypass in contrib/cirrus/cirrus_yaml_test.py for this name.
         # - podman_machine
         - local_system_test
         - remote_system_test