Use mamba solver instead of conda solver

[glatterf42]

Since we pinned the message-ix version to help the conda workflow pass, we need to keep this version number up-to-date.

The above was true in principle, but did not solve our CI conda-forge test problems. As suggested in conda/conda/issues/12741, replacing the conda solver with the mamba solver did. This even allows us to remove the message_ix version pin entirely.

How to review

• Read the diff and note that the CI checks all pass.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅

[codecov]

Codecov Report

Merging #715 (ff25045) into main (4663f1d) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #715   +/-   ##
=====================================
  Coverage   94.4%   94.4%           
=====================================
  Files         43      43           
  Lines       3448    3448           
=====================================
  Hits        3257    3257           
  Misses       191     191           

[khaeru]

Please:

1. Uncomment these lines:

   message_ix/.github/workflows/conda.yaml

   Lines 9 to 11 in 4663f1d

   	# Uncomment to debug
   	# pull_request:
   	# branches: [ main ]

2. Commit with a message like "TEMPORARY …"
3. Push.
4. Confirm that the workflow passes with this change, e.g. by leaving a comment or updating the PR description like "Note here that the jobs all pass with this change."
5. Drop the temporary commit from the branch, e.g. git reset --hard HEAD~1 && git push --force.

((5) isn't necessary if the PR will be squash-merged.)

[glatterf42]

The tests all pass using the mamba solver. Thus, I will

• Change the description of this PR
• Reset all commits and introduce just the changes we want to keep
• Open a new issue to deal with the cache saving problem