Improve Conda packaging

There a couple of issues pending with the Conda packages:

• Update the Cos7 image from upstream and test Lima.Core build
• Update our Gitlab CI runner to use the Cos7
• Fix boost-cpp package conflict with the package built for Lima2 (with MPI enabled), see on-going discussion
• Switch to mambabuild

Note: CentOS 7 docker images ad mambabuild are now the default on Conda Forge since December 2021.