fix: set runner_name in deregister verify.yml

The deregister scenario's verify.yml was missing the runner_name var,
which is required because gitea_runner_data_dir depends on it via
defaults/main.yml. Without it, the verify phase fails with
"'runner_name' is undefined".

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

ansible/roles/gitea-runner/molecule/deregister/verify.yml

@@ -2,6 +2,8 @@
 - name: Verify
   hosts: all
   become: true
+  vars:
+    runner_name: "deregister-test-runner"
   pre_tasks:
     - name: Load role defaults
       ansible.builtin.include_vars: