[llvm] [lit/tests] Replace 'env -u' with more portable construct

Set environment variables to empty values rather than attempting
to unset them via 'env -u', in order to fix NetBSD test regression
caused by r366980.  POSIX does not guarantee that env(1) supports '-u'
option, and indeed NetBSD env(1) does not support it.

Differential Revision: https://reviews.llvm.org/D65335

llvm-svn: 367123