[lit] Fix the way we check if an environment var is set

The old method would throw a KeyError.

llvm-svn: 370138

lldb/lit/Suite/lit.cfg

@@ -39,7 +39,7 @@ def find_shlibpath_var():
   elif platform.system() == 'Windows':
     yield 'PATH'
 
-if not config.environment['DYLD_INSERT_LIBRARIES']:
+if 'DYLD_INSERT_LIBRARIES' in os.environ:
   # Shared library build of LLVM may require LD_LIBRARY_PATH or equivalent.
   # This clashes with DYLD_INSERT_LIBRARIES which is needed on Darwin.
   for shlibpath_var in find_shlibpath_var():