ResolvePackageNotFound error in Ubuntu/Linux

Not sure if it's a PyPlot-specific problem (could be caused by `PyCall.jl` or `matplotlib` itself I suppose)

Upon a fresh install of the latest release of PyCall (setting `ENV["PYTHON"]=""` to use a separate Julia miniconda environment), it fails to install certain packages, including matplotlib. Trying to install `PyPlot.jl` (or even manually activating the environment and performing `conda install matplotlib`) returns the following error:
```bash
Solving environment: unsuccessful initial attempt using frozen solve. Retrying with flexible solve.
Solving environment: unsuccessful attempt using repodata from current_repodata.json, retrying with next repodata source.

ResolvePackageNotFound:
- conda==23.1.0
```

Reproducible across multiple Ubuntu machines running 18.04, 20.04, or 22.04

Start by reproducing a fresh PyCall.jl miniconda setup with ENV["PYTHON"]="" and the PyPlot.jl or matplotlib installation described in the report. Inspect the conda environment's package resolution and determine what change would allow matplotlib to install without the conda==23.1.0 ResolvePackageNotFound error across the reported Ubuntu versions.

Vom Indexierungsmodell aus dem Issue-Text verfasst.