Avoid default Anaconda channel for environment

[peanutfun]

Changes proposed in this PR:

• Set nodefaults channel in environment specs

This PR fixes https://github.com/CLIMADA-project/climada_python/issues/843 and follows-up on the equivalent PR in the Core package, https://github.com/CLIMADA-project/climada_python/pull/845.

PR Author Checklist

• [x] Read the Contribution Guide
• [x] Correct target branch selected (if unsure, select develop)
• [x] Descriptive pull request title added
• [x] Source branch up-to-date with target branch
• [ ] Documentation updated
• [ ] Tests updated
• [x] Tests passing
• [x] No new linter issues
• [x] Changelog updated

PR Reviewer Checklist

• [ ] Read the Contribution Guide
• [ ] CLIMADA Reviewer Checklist passed
• [ ] Tests passing
• [ ] No new linter issues

[emanuel-schmid]

🙌 Thanks!