ci: Add job for Mamba installation

[peanutfun]

Changes proposed in this PR:

• Add a CI job script for a regular installation test of CLIMADA via Mamba
  • Install CLIMADA, and import it in Python. No tests are executed
• Execute this job every day at 6 am or manually upon user request (workflow_dispatch)

Notes:

• Remove on: push before merging!
• Unit test results can get published to the wrong test suite (Test Install / Core / Unit Test Results instead of GitHub CI / Core / Unit Test Results). This is a known issue that currently cannot be fixed: Publishes to random workflow EnricoMi/publish-unit-test-result-action#12

This PR fixes #

PR Author Checklist

• Read the Contribution Guide
• Correct target branch selected: main because scheduled/triggered jobs only work from default branch
• Descriptive pull request title added
• Source branch up-to-date with target branch
• Documentation updated
• Tests updated
• Tests passing
• No new linter issues
• Changelog updated

PR Reviewer Checklist

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

[spjuhel]

Looks good to me!

We'll have to remember to update the matrix when dropping/supporting python versions