core: strv_env_clean() modifies in-place, no need to set buf to itself

src/core/exec-invoke.c

@@ -5254,7 +5254,7 @@ int exec_invoke(
                 *exit_status = EXIT_MEMORY;
                 return log_oom();
         }
-        accum_env = strv_env_clean(accum_env);
+        strv_env_clean(accum_env);
 
         (void) umask(context->umask);