ci: lock herokuish buildpack url at ref
Builds are failing due to what must be a recent change, see https://gitlab.com/gitlab-org/cluster-integration/auto-build-image/-/jobs/1387085411
Users report this ref working for them, see https://github.com/HashNuke/heroku-buildpack-elixir/issues/194#issuecomment-861680496
Edited by Hordur Freyr Yngvason