appveyor builds fail due to conda out of memory

[KrisThielemans]

For instance https://ci.appveyor.com/project/KrisThielemans/stir/builds/51578404/job/ogr2l6alpjp55lrs

for /D %%d in (C:\Miniconda*-x64) do set MINICONDA=%%d
echo Using Miniconda %MINICONDA%
Using Miniconda C:\Miniconda3[8](https://ci.appveyor.com/project/KrisThielemans/stir/builds/51578404/job/ogr2l6alpjp55lrs#L8)-x64
set PATH=%MINICONDA%;%MINICONDA%\Scripts;%MINICONDA%\Library\bin;%PATH
conda install -c conda-forge  -yq libparallelproj swig numpy pytest matplotlib
Collecting package metadata (current_repodata.json): ...working... done
Solving environment: ...working... failed with initial frozen solve. Retrying with flexible solve.
Solving environment: ...working... failed with repodata from current_repodata.json, will retry with next repodata source.
Collecting package metadata (repodata.json): ...working... done
Solving environment: ...working... *** picosat: out of memory in 'resize'
Command exited with code -1073740791