bigquery: set exists_ok=False when creating staging tables

This fixes two issues:

- `exists_ok=True` means we send an API call and don't wait to see
  if it succeeded. This causes a race condition where the table isn't
  ready when we attempt to use it to store rows.
- If the table already exists, we will run into issues when we try and
  run the `MERGE` query to move the staging table into the main table.

Author: cgsheeh

stats.py

@@ -538,7 +538,7 @@ def create_staging_tables(
     return {
         sql_table_id(table): bq_client.create_table(
             bigquery.Table(staging_table_id(sql_table_id(table)), schema=table.schema),
-            exists_ok=True,
+            exists_ok=False,
         )
         for table in tables.values()
     }