[lit] Use a more appropriate default for host_library_env

Author: DougGregor

test/lit.cfg

@@ -2485,7 +2485,7 @@ if hasattr(config, 'target_library_path_var'):
           "/usr/bin/env " +
           construct_library_path_env([config.swift_host_lib_dir]))
 else:
-  host_library_env = ""
+  host_library_env = "env"
 
 config.substitutions.append(('%host-library-env', host_library_env))