nspawn: require /etc/os-release only for init

/etc/os-release is expected for the case for booting a full system, and
need not be required for thin container execution.

src/nspawn/nspawn.c

@@ -1200,7 +1200,7 @@ int main(int argc, char *argv[]) {
                 goto finish;
         }
 
-        if (path_is_os_tree(arg_directory) <= 0) {
+        if (arg_boot && path_is_os_tree(arg_directory) <= 0) {
                 log_error("Directory %s doesn't look like an OS root directory (/etc/os-release is missing). Refusing.", arg_directory);
                 goto finish;
         }