Can't downgrade python: "No module named 'distutils.spawn'"

Issue:

I'm not sure whether it is a CF or conda issue, but it only occurs after running `conda update --all` with conda-forge channel added.

Here is a `Dockerfile` to reproduce:

```dockerfile
FROM continuumio/miniconda3

RUN conda config --add channels conda-forge && \
conda config --set channel_priority strict && \
conda update --all

RUN conda install python=2.7
```

Fails with:
```
Preparing transaction: ...working... done
Verifying transaction: ...working... done
Executing transaction: ...working... done
ERROR conda.core.link:_execute(637): An error occurred while installing package 'conda-forge::ipaddress-1.0.22-py_1'.
ModuleNotFoundError("No module named 'distutils.spawn'")
Attempting to roll back.

Rolling back transaction: ...working... done

ModuleNotFoundError("No module named 'distutils.spawn'")
()
```

The failing package varies based on the python version downgraded to, with `python=3.6`:

```
ERROR conda.core.link:_execute(637): An error occurred while installing package 'conda-forge::tqdm-4.32.2-py_0'.
ModuleNotFoundError("No module named 'distutils.spawn'")
Attempting to roll back.

Rolling back transaction: ...working... done

ModuleNotFoundError("No module named 'distutils.spawn'")
()
```

Tried both with and without `conda config --set channel_priority strict`, same outcome.