Fix `tls13-compat.sh` changing based on exactly how
`generate_tls13_compat_tests.py` was run (e.g. from which directory). This
made `check-generated-files.sh` behave differently from `make`. The script
has no official variations of the content of its output file, so we don't
need to record the full command line.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>