Update build-environment image to use Terraform 1.8.0

[SarahFrench]

The intention of this change is to enable running provider-defined functions in VCR, as they're a new feature in 1.8.0

This PR is blocked until the GA release of Terraform 1.8.0. We're waiting for the GA release before making this change, instead of swapping to use a release candidate now and swapping to the GA release just a few weeks after.

Release Note Template for Downstream PRs (will be copied)

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

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[SarahFrench]

This is blocked until bugs in 1.8 are resolved

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR hasn't generated any diffs, but I'll let you know if a future commit does.

[SarahFrench]

This comment has some useful context about updating the version of Terraform used during testing: https://github.com/hashicorp/terraform-provider-google/issues/18109#issuecomment-2107736745

This PR is the final step for upgrading the Terraform version used for testing the provider. TeamCity is currently using 1.8.3 and the last step is making the automated tests on PRs use the same version.

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[SarahFrench]

I realised that Scott is on vacation, so re-assigning review.

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