I guess that's one way to install large games if space on the system partition wouldn't normally allow it, then. But it does seem like a rather blunt solution and with possible issues, since anything else that'd try to use TEMP during that time would also write to the new path and look there for files previously written to TEMP, which won't be there, and those written there won't be back in the regular location after the variable is set back to the default value.
If adding an advanced option to the installer GUI for Temp location is out of the question, would be nice if there'd at least be a command line parameter, /TEMP="path", but doesn't seem to be available either...

* Inno-Extract - Downloads

* Inno-Extract - Manual Page

* Youtube Video - How to Use Inno-Extract

Inno-Extract is a command line tool, meaning that you must run CMD or Powershell (Windows) or Bash (Linux) in order to use this program.

When running Inno-Extract without arguments, the game folder(s) and file(s) will be extracted to the current directory. To avoid this scenario, use the -d switch to specify a destination directory. If you use the -d switch and the destination directory does not exist, the -d switch will create it. However, the parent directory must exist or extracting will fail.