Migrate Climpact to plugin template

In this MR, we focused on migrating Climpact to the latest version of the plugin template.

The key changes include:

1. We've updated the plugin to be able to read environment variables to be used in CI as well. https://gitlab.dkrz.de/ch1187/plugins4freva/climpact/-/blob/new-migrate/src/climpact/swift.py?ref_type=heads#L175-183

2. To align with the practices used in CCP, we defined an environment variable to avoid the creation of a scratch directory during CI runs. https://gitlab.dkrz.de/ch1187/plugins4freva/climpact/-/blob/new-migrate/ProcessClimateData.py?ref_type=heads#L453

func_test: https://ch1187.gitlab-pages.dkrz.de/-/plugins4freva/climpact/-/jobs/438100/artifacts/build/func_test/index.html

docs: https://ch1187.gitlab-pages.dkrz.de/-/plugins4freva/climpact/-/jobs/438102/artifacts/docs/_build/html/index.html

Closes #5 (closed)