Simplify conda set up for macOS platform test
Description
This MR simplifies (a little) the setup for the conda-based macOS platform tests, to get past the silent mamba
failure exhibited in, e.g, https://git.ligo.org/lscsoft/lalsuite/-/jobs/2156457.
API Changes and Justification
Backwards Compatible Changes
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This change does not modify any class/function/struct/type definitions in a public C header file or any Python class/function definitions -
This change adds new classes/functions/structs/types to a public C header file or Python module
Backwards Incompatible Changes
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This change modifies an existing class/function/struct/type definition in a public C header file or Python module -
This change removes an existing class/function/struct/type from a public C header file or Python module
If any of the Backwards Incompatible check boxes are ticked please provide a justification why this change is necessary and why it needs to be done in a backwards incompatible way.
Review Status
Please provide details on any reviews related to this change and and the associated reviewers.