CI: use mambaforge, uninstall pocl before doc build

[matthiasdiener]

The doc CI error is likely caused by https://github.com/pocl/pocl/blob/569885d1a5e3b5abbaa40eb7a3061a51d35eada4/lib/CL/devices/common.c#L1198.

Using mambaforge for the CI install (like in emirge) massively speeds up installation time, from ~1hr to ~4 min.

Questions for the review:

• [ ] Is the scope and purpose of the PR clear?
  • [ ] The PR should have a description.
  • [ ] The PR should have a guide if needed (e.g., an ordering).
• [ ] Is every top-level method and class documented? Are things that should be documented actually so?
• [ ] Is the interface understandable? (I.e. can someone figure out what stuff does?) Is it well-defined?
• [ ] Does the implementation do what the docstring claims?
• [ ] Is everything that is implemented covered by tests?
• [ ] Do you see any immediate risks or performance disadvantages with the design? Example: what do interface normals attach to?