Add additional environment variables to MSI installers

### Checklist

- [x] I added a descriptive title
- [x] I searched open requests and couldn't find a duplicate

### What is the idea?

In the PR https://github.com/conda/constructor/pull/1277 it was noted that environment variables such as `INSTALLER_PATH` and `INSTALLER_PLUGINSDIR` are not set by MSI installers (but are for EXE).
These should also be set similarly for MSI Installers.

### Why is this needed?

To create a similar UX between the two Windows installer types, especially since installer builders may have pre/post uninstall/install scripts that depend on these.

### What should happen?

Environment variables `INSTALLER_PATH` and `INSTALLER_PLUGINSDIR` should be set. The `INSTALLER_PATH` is straight-forward but the variable `INSTALLER_PLUGINSDIR` needs some additional discussion since it is NSIS specific.

### Additional Context

_No response_

Start by comparing the MSI installer handling with the EXE behavior referenced in PR 1277. Determine how INSTALLER_PATH is derived and resolve the expected MSI meaning of INSTALLER_PLUGINSDIR, then verify that both environment variables are available to relevant installer scripts for each Windows installer type.

Written by the indexing model from the issue text.