Address mamba v2 incompatibilities

[jaimergp]

Closes #366

Mamba v2 compatibility needs:

• Setting MAMBA_ROOT_PREFIX to the base env location.
• Using mamba env create instead of mamba env update if the target env doesn't exist. conda does create the target env with env update if it didn't exist 🤷
• On Unix, symlink bin/mamba to condabin so it can be found in PATH later (same as Put mamba under condabin/ too conda-forge/mamba-feedstock#251).
• On Windows, write the condabin/mamba.bat file from mamba 1.x, adjusting the path to mamba.exe as necessary.

[jaimergp]

Ok, this seems to work mostly in Unix, but mamba v2 gets "stuck" for Windows in this part:

...
  Linking requests-2.32.3-pyhd8ed1ab_0
  Linking jsonschema-4.23.0-pyhd8ed1ab_0
  Linking requests-toolbelt-1.0.0-pyhd8ed1ab_0
  Linking nbformat-5.10.4-pyhd8ed1ab_0
  Linking anaconda-client-1.12.3-pyhd8ed1ab_1
  Warning: warning  libmamba draining failed An existing connection was forcibly closed by the remote host.
  
  warning  libmamba draining failed An existing connection was forcibly closed by the remote host.

[jaimergp]

Disabling PYC compilation unlocks the hang, but then it hangs again during the pip installs. Seems that in both cases a hung subprocess on Windows is involved. I guess these code blocks:

• https://github.com/mamba-org/mamba/blob/31b23473e059b47c03b065e5592ea5a7cf77fc9b/libmamba/src/api/pip_utils.cpp#L173
• https://github.com/mamba-org/mamba/blob/31b23473e059b47c03b065e5592ea5a7cf77fc9b/libmamba/src/core/transaction_context.cpp#L319

[jaimergp]

@conda-incubator/setup-miniconda, ready for review!