"signatures" did not exist on "v11.4.11"
CI: Add COMPILE_ASSETS to cng build trigger
Add `COMPILE_ASSETS=true` to CNG build trigger. This stems from https://gitlab.com/charts/gitlab/issues/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.