Replace `railpack_environment_variables_collection()` helper (which returned
pre-filtered Eloquent attribute collections) with inline queries on
`environment_variables()` / `environment_variables_preview()` filtered by
`is_buildtime`. This ensures Railpack build variables are sourced from the
same query path as the rest of the deployment pipeline and avoids relying on
a now-removed accessor that silently included all railpack vars regardless of
build context.