test: Use libmambapy 2.0.6.rc2

[jjerphan]

Description

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[jjerphan]

There are errors because the root prefix cannot be determined.

MAMBA_ROOT_PREFIX can be set to do so. Previously it was always set thanks to a profile script wrongly shipped in libmamba, but this changed with mamba-org/mamba#3723.

The error message must be rewritten entirely because this is not clear at all to me.

Now: should we set it instead of the user or not? I still think that the value of MAMBA_ROOT_PREFIX as set by the profile script is wrong if mamba is installed in another environment than the base environment

[jaimergp]

Sure, we can patch it at runtime and set it before importing libmamba (assuming the env var set in os.environ is seen by C++?). I am more intrigued by the need for that value. If it can't be detected, shouldn't the library detect its own location and pick the relevant parent directory? Or even better: should it be specified explicitly as a parameter in the necessary API calls instead of depending on an environment variable?

[jjerphan]

Currently the location of mamba (which might not be installed) is used to locate the root prefix if the environment variable is not set, but we could use the location of libmamba instead.