Fix typo in __osx spec

[maresb]

I randomly noticed that the variable here was MACOS_DEPLOYMENT_TARGET instead of MACOSX_DEPLOYMENT_TARGET set here. I'm not bumping a build number because I think the default should handle it, so I think there's no change to the build.

Also, I'm pretty confused what's going on since I only see this being built for linux-64. But hey, I noticed this and thought I should point it out.

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 @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

[conda-forge-webservices]

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.

[timfel]

Thanks! I need to update the build scripts anyway, and this finally prods me to fix and enable the macos builds :)