[CI] Exception on teardown of PYNQ driver

[fpjentzsch]

We often get multiple instances of

[run_0] Exception ignored in: <function PynqBuffer.del at 0xffffa9fd1900>
[run_0] Traceback (most recent call last):
[run_0] File "/usr/local/share/pynq-venv/lib/python3.10/site-packages/pynq/buffer.py", line 63, in del
[run_0] File "/usr/local/share/pynq-venv/lib/python3.10/site-packages/pynq/buffer.py", line 82, in freebuffer
[run_0] TypeError: 'NoneType' object is not callable

when the driver on our CI PYNQ boards exits.