CI: Skip tests that require Mitogen when Ansible strategy is linear

The Van_* GitHub Actions jobs (corresponding to Tox factor strategy_linear,
environment variable ANSIBLE_STRATEGY=linear) were failing inside Mitogen
modules, which they should not touch. The jobs are intended as a cross
validation of the test suite, they should only fail if Ansible itself has a
problem.

tests/ansible/integration/ssh/variables.yml

@@ -14,6 +14,8 @@
       -o "ControlPath /tmp/mitogen-ansible-test-{{18446744073709551615|random}}"
 
   tasks:
+    - include_tasks: ../_mitogen_only.yml
+
     - name: ansible_user, ansible_ssh_private_key_file
       shell: >
         ANSIBLE_ANY_ERRORS_FATAL=false

tests/ansible/integration/strategy/mixed_vanilla_mitogen.yml

@@ -2,6 +2,8 @@
 - name: integration/strategy/mixed_vanilla_mitogen.yml (linear->mitogen->linear)
   hosts: test-targets[0]
   tasks:
+    - include_tasks: ../_mitogen_only.yml
+
     - connection: local
       environment:
         ANSIBLE_PYTHON_INTERPRETER: "{{ ansible_playbook_python }}"