Update conda cache to include new package format

[SimonCW]

Hey @klamann, @carsten-bo

Conda is using a new file format for downloading most packages, hence, these packages were not cached in the old config. I updated accordingly and caching should now work again.

I wanted to also fix everything to use mambaforge and mamba run instead of activating an environment but I've seen that both are already worked on.

See about new .conda:
https://conda.io/projects/conda/en/latest/user-guide/configuration/use-condarc.html#force-conda-to-download-only-tar-bz2-packages-use-only-tar-bz2

Took the paths from here:
https://damiankula.com/using_conda_cache_in_gitlabci.html

cc @ChrsBaur

[SimonCW]

I assume the failed check will be fixed after the updates to mambaforge have been merged.