Application _LOCATION environment variable being being overridden in rez environment on Windows

@eoyilmaz Added rez supported was added in #539, but this only seems to be for Linux.
Currently the *_LOCATION environment variable is being overridden in the cgru/software_setup scripts.

For example, when launching Houdini from Keeper that was started from a rez environment I have HOUDINI_LOCATION already set. So

cgru/software_setup/setup_houdini.cmd

Line 4 in fd58731

    
           For /F "Tokens=*" %%I in ('dir /b "C:\Program Files\Side Effects Software\Houdini 1*"') Do set "HOUDINI_LOCATION=C:\Program Files\Side Effects Software\%%I"

Should become something like

IF NOT DEFINED HOUDINI_LOCATION (
    For /F "Tokens=*" %%I in ('dir /b "C:\Program Files\Side Effects Software\Houdini 1*"') Do set "HOUDINI_LOCATION=C:\Program Files\Side Effects Software\%%I"
)

@Anti-Distinctlyminty would you like to implement the Windows part.

I can give it a go. I cannot test all the applications as I do not have access to them all, but I will do what I can.