bootctl: additional fixes for local/global UKI PE addons

[esposem]

Fix various memory leaks and names used in https://github.com/systemd/systemd/pull/28761.

[esposem]

@poettering @bluca

[bluca]

../src/shared/bootspec.c:1498:44: error: unused variable 'ts' [-Werror,-Wunused-variable]
                _cleanup_strv_free_ char **ts = NULL;