fix: MACOSX_DEPLOYMENT_TARGET not protected in all uses

[henryiii]

Broke scikit-build, which requires that -DCMAKE_OSX_DEPLOYMENT_TARGET= is not empty.

Checklist

• [x] Used a personal fork of the feedstock to propose changes
• [x] 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.

This was discovered in https://github.com/scikit-build/scikit-build/issues/911. I just copied the existing protection from a few lines above: https://github.com/conda-forge/clang-compiler-activation-feedstock/blob/d69b84b89b707760e3998968046cdd44cbc2e23b/recipe/activate-clang.sh#L101-L103

[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) and found it was in an excellent condition.

[isuruf]

Thanks