Pin conda-build to 3.28.4

[SilkeSchomann]

Description of work

Summary of work

I think we need to pin this to 3.28.4 to avoid problems with conda index.

To test:

Observe the branch builds properly: https://builds.mantidproject.org/job/build_packages_from_branch/748/

This does not require release notes because this is not visible to an external user

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[jhaigh0]

Can we make sure there's an issue to unpin conda-build?

[jhaigh0]

Also, this should be pointed into release-next

Changed it

[jhaigh0]

Seemed to be an unrelated test failure on Linux, trying a rerun...