CI: set TF_ACC_TERRAFORM_VERSION in workflow instead of repo variable

[BBBmau]

Description

configuration variables have a limited scope, preventing them from being used in pull requests. This causes the Deferred Actions workflow to fail since ${{ vars.TERRAFORM_VERSION_EXP }} doesn't return the version but instead just an empty string. This is a known issue across github: https://github.com/orgs/community/discussions/44322

This is seen in a recent review of a PR: https://github.com/hashicorp/terraform-provider-helm/actions/runs/9781860920/job/27387198282?pr=1247

Solution is to explicitly set it in the workflow rather than rely on a repo variable.

The same fix can be found in the kubernetes provider as well: hashicorp/terraform-provider-kubernetes#2550

Acceptance tests

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Release Note

Release note for CHANGELOG:

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References

Community Note

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