No way to specify `conda_build_config.yaml` from `settings.ini`

How to reproduce:
1. make a new `nbdev` project.
2. add a cell
> ```python
> #| export
> import numpy
> ```
3. add `requirements = numpy>=1.22` in the `settings.ini` file
4. run `nbdev_release_both --build_args '-c conda-forge`

This will result in :
```shell
conda build --output-folder out --no-anaconda-upload -c conda-forge
No numpy version specified in conda_build_config.yaml. Falling back to default numpy value of 1.21
```

Start by reproducing the issue with a new nbdev project, settings.ini, and the nbdev_release_both command shown. Trace how settings.ini and the -c build argument reach conda build; done means the configured conda_build_config.yaml or equivalent version setting is honored instead of falling back to the default numpy version.

Written by the indexing model from the issue text.