[Fleet] Fix wrong policy_id in dev tools flyout request (elastic#176729)

Fixes elastic#172798

## Summary
Small bug fix: when adding a new integration, the `policy_id` in dev
tools flyout request was not correct

Steps to reproduce:

- Add one agent policy to the stack
- Go to any integration and navigate to `add integration` page
- Switch to `Existing hosts` and select an existing policy. 
- Open `Preview API request`. The policy id in the preview should be the
one from the selected policy
- Switch back to `New hosts`
- Open `Preview API request`. The policy id should be
`<agent_policy_id>` again (not the one from the previously selected
policy)

![Screenshot 2024-02-12 at 17 56
45](https://github.com/elastic/kibana/assets/16084106/6f2e5085-7418-4fef-ac4e-79b57d551722)

x-pack/plugins/fleet/public/applications/fleet/sections/agent_policy/create_package_policy_page/single_page_layout/hooks/devtools_request.tsx

@@ -48,7 +48,7 @@ export function useDevToolsRequest({
           newAgentPolicy,
           withSysMonitoring && !packagePolicyIsSystem
         )}\n\n${generateCreatePackagePolicyDevToolsRequest({
-          ...packagePolicy,
+          ...{ ...packagePolicy, policy_id: '' },
         })}`,
         i18n.translate(
           'xpack.fleet.createPackagePolicy.devtoolsRequestWithAgentPolicyDescription',