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
https://github.com/coder/coder/pull/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 https://github.com/coder/internal/issues/607
Kacper Sawicki