Enable assisted installer so per-machine can be selected

[philrz]

https://www.electron.build/configuration/nsis describes the options being used here. Up until now we've always gone with the electron-builder default of a one-click, per-user install. With the combination I'm using in this PR, the user will be prompted as shown in the following video (made with a Dev build from this Actions run), such that they can choose the per-machine install. After selecting that install option I logged out and logged in as a different user and confirmed I saw the Zui icon on its desktop and could launch the app ok. The app binaries in that case end up in %ProgramFiles%\Zui (i.e., C:\Program Files\Zui) rather than the %USERPROFILE%\AppData\Local\Programs\Zui that we're accustomed to seeing with the per-user install.

Test.mp4

Since Zui Insiders derives its electron-builder config from Zui, I also made a Dev build in this Actions run and confirmed that also worked fine with per-machine install.

If this merges, I'll update the docs in #2531 to mention this as an alternate install option and filesystem path.

For #2685

[philrz]

This morning I also used my personal fork to confirm that this plays nicely with auto-update, and the tests were successful. A summary:

• Installed a test v1.0.1 machine-wide as an Admin user with a v1.0.2 available on GitHub as the latest. Still logged in as the Admin user, I was eventually prompted to install the newer release. After I clicked Restart I was once again shown the assisted installer and once again picked the machine-wide install. Logging out & logging in as my non-Admin user, the Zui shortcut on the desktop launched the app and it went to the v1.0.2 app as one would expect.

• Installed a test v1.0.1 machine-wide as an Admin user with a v1.0.2 available on GitHub as the latest. Once the v1.0.1 install completed, I immediately logged out and logged in as my non-Admin user, clicked the desktop shortcut to launch the v1.0.1 app, and was eventually prompted to install the newer release. After I clicked Restart I was once again shown the assisted installer and once again picked the machine-wide install, but was immediately prompted to enter Admin credentials which I canceled out of since I was a non-Admin user. At that point it brought me back to the assisted installer and I was able to pick the single-user install, and that went through fine and that user could now run v1.0.2. This user now had duplicate Zui shortcuts on the desktop which is indeed potentially confusing, but I see that more as an IT issue. At that point I logged out and switched over to my Admin user whose desktop shortcut was confirmed to still launch v1.0.1. As that Admin user I uninstalled Zui and then switched back to my non-Admin user who now had the single desktop shortcut to their personal install of v1.0.2, which still ran fine.

[jameskerr]

Very cool Phil!