19831: dist/tools/usb-serial: call ttys.py with its path [backport 2023.07] r=benpicco a=maribu

# Backport of #19823

### Contribution description
This PR provides a fix for #19805
When calling `SERIAL=xxx make flash` the command failed because the script `dist/tools/usb-serial/find-ttys.sh` tries to call `ttys.py` but it doesn't find it. So pass the full path to `ttys.py` to get rid of the problem.

### Testing procedure
Connect at least two boards to your computer
run `make list-ttys ` to get the ID of your boards
Try to flash one of them with (replace SERIAL with the one you get with the previous command):
`BOARD=xxx SERIAL=xxx make -C tests/leds flash`
It fails on master.
It should works with this PR.

### Issues/PRs references

Fixes #19805


Co-authored-by: Dylan Laduranty <[email protected]>