To avoid `This environment is externally managed` error when running
`pip install` inside a container as the default user (root).

See https://gitlab.alpinelinux.org/alpine/aports/-/issues/16108

The current approach requires a pre_build_script configured in the
runner to source $ENV. The new approach is more universal.

It's not used and removal allows simplification.

It's much smaller.

Keep it only in ci images.