No default value for the TMP variable available

It appears that ReactOS doesnt define a default place for temporary files at the
moment.

Even if there is code to make use of it (GetTempPathW in
reactos/lib/kernel32/file/curdir.c),
the lack of a default value makes some installers like Winamp fail because they
cant write temporary files in any place.

Manually setting a value makes the installer get further.
set TMP=C:\Temp
set TEMP=C:\Temp