ProcessStub: Preserve the TERM environment variable

* If Creator is not executed from terminal, this value is missing.
* If it is executed from a terminal of another type than the stub
  it might be wrong.

Change-Id: Ia6bd1dd59be04e01398ed5cdffff3080539b10ef
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>