terraform: fix GCP SQL db instance deletion

[bobbyiliev]

As per: MaterializeInc/materialize-terraform-self-managed#178

[github-actions]

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[def-]

Can you also reenable the test in ci/nightly/pipeline.template.yml by removing the skip?

[bobbyiliev]

Can you also reenable the test in ci/nightly/pipeline.template.yml by removing the skip?

ah yes, good point!

[def-]

Needs a rebase for the postgres-cdc issue, it's fixed on main.