Provide post-link scripts for subpackages (version 31)

[stuarteberg]

This is the same as #19, but backported to the ngspice-31 branch.

Provide post-link scripts for subpackages (ngspice-lib and ngspice-exe), not just the full metapackage

Checklist

• [ ] Used a 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-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.