Enable MPI build for osx-arm64

[xiki-tempula]

The MPI arm64 build won't build on the CI which I guess because it runs on a intel chip Mac and trying to cross-compile it to be arm64. I have used my M1 mac to build the conda package locally and it does work.

Checklist

• [ ] Used a personal fork of the feedstock to propose changes
• [ ] Bumped the build number (if the version is unchanged)
• [ ] Reset the build number to 0 (if the version changed)
• [ ] Re-rendered with the latest conda-smithy (Use the phrase code>@<space/conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• [ ] Ensured the license file is being packaged.

[conda-forge-webservices[bot]]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.

[xiki-tempula]

@conda-forge-admin, please rerender

[mabraham]

I see the build image has https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=1051838&view=logs&j=5e1269ef-6c32-5a63-0f37-731563990088&t=60fff8cd-70dd-5bee-bfd3-a4b49a609b49&l=1144 openmpi 5.0.5, so we need to find where that's installed and teach the cmake call how to find it there.