Build on comment definition added to BK pull request definition. (#218)

Author: mashhurs

.buildkite/pull-requests.json

@@ -8,6 +8,7 @@
       "allowed_list": ["dependabot[bot]", "mergify[bot]", "elastic-vault-github-plugin-prod[bot]"],
       "set_commit_status": true,
       "build_on_commit": true,
+      "build_on_comment": true,
       "trigger_comment_regex": "^(?:(?:buildkite\\W+)?(?:build|test)\\W+(?:this|it))",
       "always_trigger_comment_regex": "^(?:(?:buildkite\\W+)?(?:build|test)\\W+(?:this|it))",
       "skip_target_branches": [],