use mamba in docker construction
Created by: RoyStegeman
Use mamba to build the docker to avoid passing the 6hour limit on the github CI
Since I force pushed it's not longer visible, but using mamba reduce the time of the "Build docker image and export environment" step to 3 minutes: https://github.com/NNPDF/nnpdf/actions/runs/6889900597/job/18741833824