Add additional environment variables to MSI installers

[lrandersson]

Checklist

• I added a descriptive title
• I searched open requests and couldn't find a duplicate

What is the idea?

In the PR #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

[marcoesters]

INSTALLER_PLUGINSDIR is NSIS-specific to host temporary files it needs to render certain things like images. I don't think there is a briefcase/MSI equivalent.