style(bigquery): add code samples to lint check

[plamut]

Closes #9081.

This PR makes sure that the code samples, too, undergo the lint check (flake8 specifically, as black already checks the samples).

After experimenting with the synth tool a bit, I realized that the BigQuery's noxfile is not generated by synth from the template, thus modified it directly.

How to test

Introduce a few flake8-specific (i.e. not relevant to black) lint errors to some of the files in docs/samples, such as an unused import, and run the lint check:

$ nox -f noxfile.py -s lint

Actual result (before the fix):
The check passes despite the lint issues.

Expected result (before the fix):
The check correctly fails and complains about the lint issues.