Change input for installer packages to .json

[dkbennett]

Update the installer build to change input of packages to include to a .json file as a build parameter.

From jonwis:

Feels like this data should be some kind of an input parameter. This mechanism - packing up MSIXes into an SFX that can install them - isn't specific to Project Reunion.

In some future update, read these from a .json file so it's not hard-coded.

Originally posted by @jonwis in https://github.com/microsoft/ProjectReunion/pull/444#discussion_r587724828