stackruntime: Pass ExperimentsAllowed into stackeval for apply

This was originally part of 7dad938fdb but
unfortunately seems to have got lost during some rebasing, or some other
similar sort of annoying reason.

This now allows the "experiments allowed" flag to propagate into the
stackeval package when we're running the apply phase, for consistency with
all of the other phases. Without this, it's possible to plan a
configuration that's participating in experiments, but then it fails in a
strange way during the apply step due to Terraform suddenly thinking it's
a stable release where experiments are disabled.

internal/stacks/stackruntime/apply.go

@@ -59,7 +59,8 @@ func Apply(ctx context.Context, req *ApplyRequest, resp *ApplyResponse) {
 		req.Config,
 		req.RawPlan,
 		stackeval.ApplyOpts{
-			ProviderFactories: req.ProviderFactories,
+			ProviderFactories:  req.ProviderFactories,
+			ExperimentsAllowed: req.ExperimentsAllowed,
 		},
 		outp,
 	)