build: stop using path pins
build: stop using path pins

Using opam path pins means that rebuilds can start with stale build
files.

Also, don't pin datakit-ci to a tag when building self-ci. This was
needed back when we didn't support dependencies between Dockerfiles,
but not now.

Signed-off-by: Thomas Leonard <ea62ade398c3addd3bcc52285a00e98f3b0c618f@docker.com>

docker/datakit,docker/datakit,docker/datakit