Fix Project Build and Deploy exit status (elastic#172113)

## Summary

The last line of the script will evaluate to false if the security label
is not set, which results in the exit status being reported as 1
(failure), but the project is actually deployed properly.


[Example](https://buildkite.com/elastic/kibana-pull-request/builds/179784#018c17c0-5dd6-4e1f-b6ef-73fa50e1de74/4142-4148)

.buildkite/scripts/steps/serverless/build_and_deploy.sh

@@ -105,3 +105,5 @@ EOF
 is_pr_with_label "ci:project-deploy-elasticsearch" && deploy "elasticsearch"
 is_pr_with_label "ci:project-deploy-observability" && deploy "observability"
 is_pr_with_label "ci:project-deploy-security" && deploy "security"
+
+exit 0;