Make scripts (mostly) POSIX compat; add shellcheck

[mbargull]

Checklist

• [x] Used a personal fork of the feedstock to propose changes
• [x] Bumped the build number (if the version is unchanged)
• [x] 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.

The activation scripts cannot be run in non-Bash-compatible shells, e.g., when run through /bin/sh//bin/dash on Debian. This is an attempt to allow support those shells.

I did not yet test these changes.

[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.