Defaults channel is not configured correctly.

### Solution to issue cannot be found in the documentation.

- [X] I checked the documentation.

### Issue

When a .condarc/condrc file with:
```
channels:
-defaults
```
is still available within the home directory/user files (e.g. because they used Anaconda before), Miniforge will also download package from the Anaconda repository. This is because the defaults channels still seem to point to the commercial repositories instead of conda-forge (default from conda package?). For our central installations, we overruled this behavior by specifying that the defaults channel should be conda-forge:
```
default_channels:
- https://conda.anaconda.org/conda-forge
```
It would be very helpful if this could also be implemented in the .condarc of the Miniforge installation. Is that possible. Now it only contains:
```
channels:
- conda-forge
```
Some of our users manually downloaded the Miniforge installation package (e.g. to install Miniforge on Windows). If there are still .condarc/condarc files left on their system, they could still be using the commercial repositories by mistake. Especially with the changes Terms and Conditions of Anaconda in mind I think this would be a useful change. Or are do you expect this is a bug instead?

### Installed packages

```shell
n/a
```

### Environment info

```shell
Without overruling defaults channel:
active environment : None
user config file : /home/user/.condarc
populated config files : /path/to/python3/3.8-miniforge/.condarc
/home/user/.condarc
conda version : 4.12.0
conda-build version : not installed
python version : 3.8.12.final.0
virtual packages : __linux=3.10.0=0
__glibc=2.17=0
__unix=0=0
__archspec=1=x86_64
base environment : /path/to/python3/3.8-miniforge (read only)
conda av data dir : /path/to/python3/3.8-miniforge/etc/conda
conda av metadata url : None
channel URLs : https://repo.anaconda.com/pkgs/main/linux-64
https://repo.anaconda.com/pkgs/main/noarch
https://repo.anaconda.com/pkgs/r/linux-64
https://repo.anaconda.com/pkgs/r/noarch
https://conda.anaconda.org/conda-forge/linux-64
https://conda.anaconda.org/conda-forge/noarch
package cache : /cadappl/python3/3.8-miniforge/pkgs
/home/user/.conda/pkgs
envs directories : /home/user/.conda/envs
/path/to/python3/3.8-miniforge/envs
platform : linux-64
user-agent : conda/4.12.0 requests/2.27.1 CPython/3.8.12 Linux/3.10.0-1160.36.2.el7.x86_64 scientific/7.9 glibc/2.17
UID:GID : 1106:1000
netrc file : None
offline mode : False

Expected/with altered defaults channel:
active environment : None
user config file : /home/user/.condarc
populated config files : /path/to/python3/3.8-miniforge/.condarc
/home/user/.condarc
conda version : 4.12.0
conda-build version : not installed
python version : 3.8.12.final.0
virtual packages : __linux=3.10.0=0
__glibc=2.17=0
__unix=0=0
__archspec=1=x86_64
base environment : /path/to/python3/3.8-miniforge (read only)
conda av data dir : /path/to/python3/3.8-miniforge/etc/conda
conda av metadata url : None
channel URLs : https://conda.anaconda.org/conda-forge/linux-64
https://conda.anaconda.org/conda-forge/noarch
package cache : /cadappl/python3/3.8-miniforge/pkgs
/home/user/.conda/pkgs
envs directories : /home/user/.conda/envs
/path/to/python3/3.8-miniforge/envs
platform : linux-64
user-agent : conda/4.12.0 requests/2.27.1 CPython/3.8.12 Linux/3.10.0-1160.36.2.el7.x86_64 scientific/7.9 glibc/2.17
UID:GID : 1106:1000
netrc file : None
offline mode : False
```