Fixes https://github.com/coder/internal/issues/1119
## Description
The `CacheTFProviders` function in `testutil/terraform_cache.go` was
only available on Linux and macOS due to the `//go:build linux ||
darwin` build tag. This caused a compile error on Windows when
`enterprise/coderd/workspaces_test.go` tried to call it:
```
enterprise\coderd\workspaces_test.go:3403:28: undefined: testutil.CacheTFProviders
```
## Changes
1. Added `testutil/terraform_cache_windows.go` with a Windows-specific
stub implementation that returns an empty string
2. Updated `downloadProviders` helper in
`enterprise/coderd/workspaces_test.go` to handle empty paths gracefully
## Behavior
- On Linux/macOS: Terraform providers are cached as before
- On Windows: Provider caching is skipped, tests download providers
normally during `terraform init`
## Testing
This should fix the Windows nightly gauntlet failure. The test will
still run on Windows, just without provider caching optimization.
Co-authored-by: blink-so[bot] <211532188+blink-so[bot]@users.noreply.github.com>
blinkagent[bot]