chore: cache terraform providers between CI test runs (#17373)

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.

testutil/cache.go

@@ -0,0 +1,25 @@
+package testutil
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// PersistentCacheDir returns a path to a directory
+// that will be cached between test runs in Github Actions.
+func PersistentCacheDir(t *testing.T) string {
+	t.Helper()
+
+	// We don't use os.UserCacheDir() because the path it
+	// returns is different on different operating systems.
+	// This would make it harder to specify which cache dir to use
+	// in Github Actions.
+	home, err := os.UserHomeDir()
+	require.NoError(t, err)
+	dir := filepath.Join(home, ".cache", "coderv2-test")
+
+	return dir
+}