We run 3 steps when a project depends on conda.

1. create the whole environment based on user's YAML file
2. run `conda install` with our own dependencies
3. run `pip install` with some of our dependencies that are not
published on conda repositories.

This commit changes this to make it in just one step (at environment
creation). To do this, it appends our own requirements to the
`environment.yml` file under `dependencies` and `dependencies.pip`
config (see https://docs.conda.io/projects/conda/en/latest/user-guide/tasks/manage-environments.html#create-env-file-manually)

It also shows the resulting `environment.yml` in the build output.

This behavior is added behind a feature flag so we can test it first.

This allow users to be able to pin their dependencies as they want
without us upgrading them in the second step. Also, this should
improve the build time, since dependencies resolution is done just
once.

Related to

* #3829
* #5631