Addresses https://github.com/coder/internal/issues/322.
This PR starts caching Terraform providers used by `TestProvision` in
`provisioner/terraform/provision_test.go`. The goal is to improve the
reliability of this test by cutting down on the number of network calls
to external services. It leverages GitHub Actions cache, which [on depot
runners is persisted for 14 days by
default](https://depot.dev/docs/github-actions/overview#cache-retention-policy).
Other than the aforementioned `TestProvision`, I couldn't find any other
tests which depend on external terraform providers.
