Can not use "conda install" in binder environment

It seems that to whatever reasons, it is not possible to install a package into the binder environment via conda.
I tried via the terminal or directly from within the jupyter notebook. I started binder by clicking one of the links from the dask-example page.

It does not really matter which package (I tried dask-sql, but also e.g. pandas, which is already installed). Both fail with:
```
Collecting package metadata (current_repodata.json): done
Solving environment: failed with initial frozen solve. Retrying with flexible solve.
Collecting package metadata (repodata.json): done
Solving environment: failed with initial frozen solve. Retrying with flexible solve.

PackagesNotFoundError: The following packages are missing from the target environment:
- nodejs==14
- numpy==1.18
```

The installation seems to work with mamba, which makes me think it is some problem in the way conda resolves the packages - but I have no idea what is going wild her.

Now comes the "funny" part: If I explicitly install
```
conda install numpy=1.18 nodejs=14
```
it actually works, even though conda list already shows me them as installed. It will downgrade some packages:

```
The following packages will be UPDATED:

certifi 2020.6.20-py38h32f6830_0 --> 2020.6.20-py38h924ce5b_2

The following packages will be DOWNGRADED:

jupyterlab 2.1.5-py_0 --> 2.1.0-py_1
pandas 1.0.5-py38hcb8c335_0 --> 1.0.0-py38hb3f55d8_0
scikit-learn 0.23.2-py38h5d63f67_2 --> 0.23.0-py38h3a94b23_0
```

After that, I can also install other packages...