[lit] Fix my earlier bogus fix to not set DYLD_LIBRARY_PATH with Asan.

My follow-up commit to mess with DYLD_LIBRARY_PATH was bogus for two
reasons:

 - The condition was inverted.
 - We were checking the OS's environment, instead of the config's.

Two wrongs don't make a right, but the second mistake meant that the
sanitizer bot passed.

llvm-svn: 370483