ci(bigquery): avoid race condition in create table by using a dataset scoped to a specific user and python version

[cpcloud]

We do this for other bigquery tests, to avoid failures like this: https://github.com/ibis-project/ibis/actions/runs/10921405259/job/30313419648.

[cpcloud]

I'll poke around the bigquery test suite and see if there are other cases where we aren't scoping to a specific user/dataset combo.

[cpcloud]

It looks like everywhere else we're using con, which is by construction scoped to a user/database
or we're making a new backend instance with the dataset_id fixture which is construction that way as well.

[cpcloud]

BigQuery tests passing locally:

…/ibis on  fix-bigquery-test is 📦 v9.5.0 via 🐍 v3.12.5 via ❄️   impure (ibis-3.12.5-env)
❯ pytest -m bigquery -n auto --dist loadgroup --snapshot-update -q
bringing up nodes...
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1720 passed, 132 skipped, 350 xfailed, 5 xpassed in 382.80s (0:06:22)