FindEnvModules: BUGFIX: env_module_swap OUTPUT_VARIBLE

Since originally added to CMake, env_module_swap(OUTPUT_VARIABLE)
did not give output since the incorrect variable name was used.

Modules/FindEnvModules.cmake

@@ -259,7 +259,7 @@ function(env_module_swap out_mod in_mod)
   )
 
   if(MOD_ARGS_OUTPUT_VARIABLE)
-    set(${MOD_ARGS_OUTPUT_VARIABLE} "${err_var}" PARENT_SCOPE)
+    set(${MOD_ARGS_OUTPUT_VARIABLE} "${tmp_out}" PARENT_SCOPE)
   endif()
   if(MOD_ARGS_RESULT_VARIABLE)
     set(${MOD_ARGS_RESULT_VARIABLE} ${tmp_ret} PARENT_SCOPE)