ci: Add COMPILE_ASSETS to cng build trigger

What does this MR do?

Add COMPILE_ASSETS=true to CNG build trigger.

This stems from charts/gitlab#937, where we needed to add asset compilation to the CNG image pipelines when using < 11.5.x. This is only needed on versions prior to 11.5, as they do not have the asset compilation container backported.

This MR is intentionally targeted only at the 11-4-stable branch and not master.

What are the relevant issue numbers?

charts/gitlab#937

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.

cc @twk3 @marin