You can use my strategy, so just install it anywhere you like and add a small batch file to run it from whatever you launch it…also add a config directory locally to that folder and if you need all things contained you would finally need to put your thumbnail image cache there…This might not be practical for a “portable” install… but you can direct the location of the kernel and thumbnail cache files… if you need them otherwise they will be created in the Windows user data as well… You can now also create a zip file of this install that you can install it as a “portable” version on any computer. As for the environment variable… if you run DT using the --configdir parameter you can use any directory you want for the config files… That is what I do for a portable version ie use that parameter in the aforementioned batch file and add a local config directory…
EDIT
Bat file is simply this
.\bin\darktable --configdir “.\config”