Add create_environment task for the RTW

[ehogan]

Add a new create_environment task / app. The create_environment task / app will use mamba to create the required environment using the environment.yml file.

Use conda to first create a minimal environment containing mamba, then use mamba to create the ESMValTool environment.

Use --prefix to install the ESMValTool environment into ${CYLC_WORKFLOW_RUN_DIR} and ensure the packages are downloaded to a temporary space.

[ehogan]

It was demonstrated via #3246 that a Docker container in the RTW works well on JASMIN. There is an approach that may work at the MO. Closing this issue in favour of #3579.