[Security Solution][Endpoint] Update the agents.queue setting for Defend Workflow buildkite jobs (#166981)

## Summary

- sets the `agents.queue` for Defend Workflows jobs to `n2-4-virt`,
since these jobs need to create and run VMs using Vagrant
- Should address failures seen in:
https://buildkite.com/elastic/kibana-on-merge-unsupported-ftrs

.buildkite/pipelines/on_merge_unsupported_ftrs.yml

@@ -88,7 +88,7 @@ steps:
   - command: .buildkite/scripts/steps/functional/defend_workflows.sh
     label: 'Defend Workflows Cypress Tests'
     agents:
-      queue: n2-4-spot
+      queue: n2-4-virt
     depends_on: build
     timeout_in_minutes: 120
     parallelism: 6

.buildkite/pipelines/pull_request/defend_workflows.yml

@@ -14,7 +14,7 @@ steps:
   - command: .buildkite/scripts/steps/functional/defend_workflows_serverless.sh
     label: 'Defend Workflows Cypress Tests on Serverless'
     agents:
-      queue: n2-4-spot
+      queue: n2-4-virt
     depends_on: build
     timeout_in_minutes: 120
     parallelism: 2