Don't use sed in _tc_activation

[manopapad]

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)

The clang activation script causes conda activation to take ~5s on my Mac laptop, which is quite annoying for those of us who want to conda activate on shell startup. The reason for this is that sed is called in a loop in _tc_activation, to split a string at the first occurrence of a comma, and sed seems to be quite slow on Mac. However, there is no reason to use sed for this; substring-substitution is a fast and portable way to achieve the same effect.

This change doesn't apply to just this repo's recipe, it could be copied onto every activation script that uses _tc_activation, but I couldn't easily find a good place to make this change globally.

[conda-forge-linter]

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.

[manopapad]

@conda-forge-admin, please rerender