CI is being fairly unreliable

[ariostas]

Recently, the CI is being fairly flaky. Usually, at least one of the test jobs randomly fails. The issue is with the conda-incubator/setup-miniconda action, as it randomly fails to set up the environment properly. It seems like other people are having the same issue here: conda-incubator/setup-miniconda#392 . They mention that explicitly specifying the mamba version fixes the issue, so I think we should just do that if this is not fixed within a couple weeks.