Adapt to Alpine's "dynamic downstream pipelines"

mrtest on Alpine is basically broken since Alpine switched to dynamic downstream pipelines in https://gitlab.alpinelinux.org/alpine/aports/-/commit/08d03b91a8680f30c23cff24f6f4be43d326636f

mentioned in commit ac1c0f50

mentioned in merge request !65 (merged)

mentioned in commit bd962f02

mentioned in commit c2c9daa0

mentioned in commit b96435f1

closed with commit b96435f1