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!

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Thanks!

[marcoesters]

PATHs are currently manipulated using Python and (soon) conda-standalone (see #1089). So, using NSIS shouldn't be needed anymore.