Fixes flaky TestWorkspaceTagsTerraform and TestWorkspaceTemplateParamsChange tests that were failing with connection reset by peer errors when downloading the coder/coder provider.

This applies the same caching solution which was done in #17373

1. Extracts provider caching logic into testutil/terraform_cache.go
2. Updates TestProvision to use the shared caching helpers
3. Updates enterprise workspace tests to use the shared caching helpers

The cache is persisted at ~/.cache/coderv2-test/ and automatically cached between CI runs via existing GitHub Actions cache setup.

Closes coder/internal#607