fix(bigquery): fix arrow deprecation warning

[plamut]

Fixes #9485.

This PR fixes a warning that can be issued when downloading results as pyarrow record batches.

The warning was caused by pyarrow, because a pyarrow schema was passed as the names argument, instead of as the schema argument, and pyarrow started raising a warning for it in version 0.15.0 (source code).

Steps to reproduce

• Make sure pyarrow 0.15.0+ is installed (there is no warning in prior versions),
• Run query_to_arrow test,

Actual result (before the fix):
A pyarrow FutureWarning is issued (as described in the issue description).

Expected result (after the fix):
No warning is issued, everything works as expected.