Address usability issue in generate-loose-foreign-key script

What does this MR do and why?

Describe in detail what your merge request does and why.

In https://gitlab.com/gitlab-org/gitlab/-/jobs/3886691360, I came across a couple of issues with the code generated for LFKs. This MR addressed those issues and closes #394860 (closed).

Patch generated by script

diff --git a/config/gitlab_loose_foreign_keys.yml b/config/gitlab_loose_foreign_keys.yml
index ca1129ef288b..2c630c9c1bed 100644
--- a/config/gitlab_loose_foreign_keys.yml
+++ b/config/gitlab_loose_foreign_keys.yml
@@ -244,6 +244,10 @@ p_ci_builds_metadata:
   - table: projects
     column: project_id
     on_delete: async_delete
+p_ci_runner_machine_builds:
+  - table: ci_runner_machines
+    column: runner_machine_id
+    on_delete: async_delete
 packages_build_infos:
   - table: ci_pipelines
     column: pipeline_id
diff --git a/db/structure.sql b/db/structure.sql
index 50cc6403eda9..a66cab5eea3b 100644
--- a/db/structure.sql
+++ b/db/structure.sql
@@ -35737,9 +35737,6 @@ ALTER TABLE ONLY dast_scanner_profiles
 ALTER TABLE ONLY vulnerability_historical_statistics
     ADD CONSTRAINT fk_rails_72b73ed023 FOREIGN KEY (project_id) REFERENCES projects(id) ON DELETE CASCADE;
 
-ALTER TABLE p_ci_runner_machine_builds
-    ADD CONSTRAINT fk_rails_72ffb51d74 FOREIGN KEY (runner_machine_id) REFERENCES ci_runner_machines(id) ON DELETE CASCADE;
-
 ALTER TABLE ONLY custom_emoji
     ADD CONSTRAINT fk_rails_745925b412 FOREIGN KEY (namespace_id) REFERENCES namespaces(id) ON DELETE CASCADE;
 
diff --git a/spec/models/ci/runner_machine_build_spec.rb b/spec/models/ci/runner_machine_build_spec.rb
index b15d232cbb56..cd5005f55548 100644
--- a/spec/models/ci/runner_machine_build_spec.rb
+++ b/spec/models/ci/runner_machine_build_spec.rb
@@ -47,4 +47,11 @@
     it { expect(partitioning_strategy.current_partitions).to include partitioning_strategy.initial_partition }
     it { expect(partitioning_strategy.active_partition).to be_present }
   end
+
+  context 'with loose foreign key on p_ci_runner_machine_builds.runner_machine_id' do
+    it_behaves_like 'cleanup by a loose foreign key' do
+      let!(:parent) { create(:ci_runner_machine) }
+      let!(:model) { create(:p_ci_runner_machine_build, runner_machine: parent) }
+    end
+  end
 end

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