samples builds failing on merge

[tswast]

==================================== ERRORS ====================================
______________ ERROR collecting samples/snippets/read_gbq_test.py ______________
ImportError while importing test module '/workspace/samples/snippets/read_gbq_test.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/local/lib/python3.9/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
read_gbq_test.py:7: in <module>
    from . import read_gbq_legacy
read_gbq_legacy.py:9: in <module>
    import pandas_gbq
E   ModuleNotFoundError: No module named 'pandas_gbq'
-------- generated xml file: /workspace/samples/snippets/sponge_log.xml --------
=========================== short test summary info ============================
ERROR read_gbq_test.py
!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!
=============================== 1 error in 0.18s ===============================
nox > Command pytest --junitxml=sponge_log.xml failed with exit code 2
nox > Session py-3.9 failed.

 Testing failed: Nox returned a non-zero exit code.

================================================================
2022-03-04T12:13:14Z: Build finished with 1
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https://source.cloud.google.com/results/invocations/39751ca1-ad47-459f-9af9-5fc16d6a8393/targets/cloud-devrel%2Fclient-libraries%2Fpython%2Fgoogleapis%2Fpython-bigquery-pandas%2Fsamples%2Fpython3.9%2Fperiodic/log

I suspect we don't have pandas-gbq in the requirements.txt. That said, maybe we want to add the environment variable to use the source version of pandas-gbq in samples tests?