You can download PSExec from Microsoft Sysinternals https://microsoft.com/sysinternals.
To get a shell like what the BigFix client uses with wow64 redirection turned off, open a command prompt and enter
psexec -i -s c:\windows\system32\cmd.exe
This opens a new command prompt window, running as the SYSTEM account. Then try running your powershell script, you should see the same sort of results as when it runs under BigFix
powershell.exe -ExecutionPolicy Bypass -File c:\temp\WindowsUnquote\WindowsUnquote.ps1
To simulate the default BigFix environment (32-bit mode), one could invoke psexec via
psexec -i -s c:\windows\syswow64\cmd.exe