Rename HELM_BUILD_TRIGGER_TOKEN_SECURITY to HELM_BUILD_TRIGGER_TOKEN

While transitioning auto-deploy branches to the Security mirror, we had a reason to keep both of these variables side-by-side. Then we removed the auto_deploy_on_security feature flag, which meant we only needed the value of the _SECURITY variable.

This commit renames the variable back to its previous name for clarity.

Closes gitlab-com/gl-infra/delivery#1228 (closed)

The variable value has already been adjusted in https://ops.gitlab.net/gitlab-org/release/tools/-/settings/ci_cd. Once this is merged we can remove the _SECURITY definition.