adbc-driver-flightsql: install protobuf for the test suite

Author: luhenry

.github/workflows/build-adbc-driver-flightsql.yml

@@ -174,9 +174,10 @@ jobs:
           # rejects the wheel as unsupported.
           pip install -q --upgrade pip
           # adbc-driver-manager/pandas/pyarrow riscv64 wheels only exist on our
-          # own registry.
+          # own registry. protobuf is upstream's own [test] extra (test_errors.py/
+          # test_incremental.py import google.protobuf directly).
           PIP_EXTRA_INDEX_URL=https://pypi.riseproject.dev/simple/ pip install -q \
-            wheelhouse/*.whl adbc-driver-manager numpy pandas "pyarrow>=14.0.1" "pytest>=9"
+            wheelhouse/*.whl adbc-driver-manager numpy pandas protobuf "pyarrow>=14.0.1" "pytest>=9"
 
           export ADBC_TEST_FLIGHTSQL_URI="grpc://localhost:41414"
           python -c "import adbc_driver_flightsql; import adbc_driver_flightsql.dbapi"