feat(blueprints): get audience from tfc environment variable

[Thomgrus]

Subject

This PR is about using Workload Identity Federation with Terraform Cloud.

Description

I propose this change because the audience in Terraform Cloud / Enterprise is already defined by the user in the TFC_WORKLOAD_IDENTITY_AUDIENCE environment variable for the workspace. So you can avoid to define it in the Terraform code as variable.

I take this opportunity to also correct a typo error in the documentation.

Thank you for the sample code, it helped me a lot to understand how to use Workload Identity Federation with Terraform Cloud.

Have a nice day ! 👋

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[averbuks]

Thank you @Thomgrus for the contribution, you are right, the var is already defined, that way provider configuration is even simpler.