Improve mamba usage in CI

- install mamba (https://github.com/mamba-org/mamba) first, and use it
to speed up installation of all other conda envs, including the one for
synthetic data. Note that currently only the dependency solve is
accelerated (mamba-org/mamba#633), but this is
sufficient for a large improvement over conda.

- Remove the --mamba flag from the conda_env_setup.sh script's
arguments, as this was taken out in NOAA-GFDL#166.

.github/workflows/mdtf_tests.yml

@@ -192,3 +192,4 @@ jobs:
       run: |
         conda activate _MDTF_base
         python -m unittest discover
+