Remove walrus operator to make 1.5 py37 compatible

[mikealfare]

Problem

We backported a change to v1.5 that used the walrus operator, which was introduces in py38. While our goal is to drop support for py37 going forward, this is a small change that would keep v1.5 py37 compatible for a bit longer.

Solution

Replace the walrus operator with an equivalent two lines.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the dbt-bigquery contributing guide.

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the dbt-bigquery contributing guide.

[mikealfare]

I converted this to a draft since we might choose to remove testing for py37 as an alternative.

[mikealfare]

We are instead choosing to remove py37 testing. This is not needed.