Fix how NSIS amends PATH on Windows

[jaimergp]

https://nsis.sourceforge.io/Setting_Environment_Variables has a big warning about this:

WARNING: Strings longer than ${NSIS_MAX_STRLEN} will get truncated/corrupted. Do NOT use this function to update %PATH%, use the EnVar_plug-in instead.

... and that's how it's being done right now. Unclear if $INSTDIR and $0 get expanded for the count, but I assume they do and on (very?) long paths, this might be an issue. This is worth an investigation and, if we need this, we would have to add the EnVar plugin to the nsis builds on conda-forge and defaults.

[github-actions]

Hi there, thank you for your contribution!

This issue has been automatically marked as stale because it has not had recent activity. It will be closed automatically if no further activity occurs.

If you would like this issue to remain open please:

1. Verify that you can still reproduce the issue at hand
2. Comment that the issue is still reproducible and include:
   - What OS and version you reproduced the issue on
   - What steps you followed to reproduce the issue

NOTE: If this issue was closed prematurely, please leave a comment.

Thanks!