fix: add Windows stub for CacheTFProviders (#20840)

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>

enterprise/coderd/workspaces_test.go

@@ -3393,6 +3393,7 @@ func workspaceTagsTerraform(t *testing.T, tc testWorkspaceTagsTerraformCase, dyn
 // This uses the shared testutil caching infrastructure to avoid re-downloading
 // providers on every test run. It is the responsibility of the caller to set
 // TF_CLI_CONFIG_FILE.
+// On Windows, provider caching is not supported and an empty string is returned.
 func downloadProviders(t *testing.T, providersTf string) string {
 	t.Helper()
 
@@ -3401,7 +3402,9 @@ func downloadProviders(t *testing.T, providersTf string) string {
 	testName := "TestWorkspaceTagsTerraform"
 
 	cliConfigPath := testutil.CacheTFProviders(t, cacheRootDir, testName, templateFiles)
+	if cliConfigPath != "" {
 		t.Logf("Set TF_CLI_CONFIG_FILE=%s", cliConfigPath)
+	}
 	return cliConfigPath
 }
 

testutil/terraform_cache_windows.go

@@ -0,0 +1,17 @@
+//go:build windows
+
+package testutil
+
+import (
+	"testing"
+)
+
+// CacheTFProviders is a no-op on Windows.
+// Terraform provider caching is only supported on Linux and macOS due to
+// platform-specific filesystem operations and Terraform's provider mirror behavior.
+// On Windows, tests will download providers normally during terraform init.
+func CacheTFProviders(t *testing.T, rootDir string, testName string, templateFiles map[string]string) string {
+	t.Helper()
+	t.Log("Terraform provider caching is not supported on Windows; providers will be downloaded normally")
+	return ""
+}