tools: ynl: add script dir to sys.path

Python options like PYTHONSAFEPATH or -P [1] do not add script
directory to PYTHONPATH. ynl depends on this path to build and run.

[1] This option is default for Fedora rpmbuild since introduction of
    https://fedoraproject.org/wiki/Changes/PythonSafePath



Signed-off-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/b26537cdb6e1b24435b50b2ef81d71f31c630bc1.1731399562.git.jstancek@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>