Add pin for fmt in conda

[G-D-Petrov]

Reference Issues/PRs

What does this implement or fix?

There is a new version of fmt that is having problems linking to spdlog.
The proposed fix is to pin fmt to the previous stable version.

Any other comments?

Checklist

Checklist for code changes...

• Have you updated the relevant docstrings, documentation and copyright notice?
• Is this contribution tested against all ArcticDB's features?
• Do all exceptions introduced raise appropriate error messages?
• Are API changes highlighted in the PR description?
• Is the PR labelled as enhancement or bug so it appears in autogenerated release notes?

[poodlewars]

Approved - this seems unavoidable. We'll have to remember to add this to the feedstock when we next do a Conda release