Pass BUILDER_REGISTRY and related CI variables as trigger variables to omnibus-gitlab-mirror
Summary
As described in !4344 (comment 366099170), we currently have BUILDER_IMAGE_REGISTRY
set as variables in the https://gitlab.com/gitlab-org/build/omnibus-gitlab-mirror project.
Proposal
As project variables, they override variables set in the CI file, which makes it difficult to test out changes that need to be pulled from registry branches.
These variables should instead be passed to the downstream mirror pipeline as trigger variables:
BUILDER_IMAGE_REGISTRY
PUBLIC_BUILDER_IMAGE_REGISTRY
BUILDER_IMAGE_REVISION
References
This came up while working on #5176 (closed).