Legacy executors are executors after all

This fixes a bug where pre-0.9 executors fail to recover after an
upgrade.

The bug is that legacyExecutorWrappers didn't get updated with
ExecStreaming function, and thus failed to implement the Executor
function. Sadly, this meant that all recovery attempts fail, as the
runtime check in
https://github.com/hashicorp/nomad/blob/b312aacbc961dfd976b6a402d22d80b45176a962/drivers/shared/executor/utils.go#L103-L110
.

drivers/shared/executor/legacy_executor_wrapper.go

@@ -36,6 +36,9 @@ type legacyExecutorWrapper struct {
 	logger hclog.Logger
 }
 
+// validate that legacyExecutorWrapper is an executor
+var _ Executor = (*legacyExecutorWrapper)(nil)
+
 func (l *legacyExecutorWrapper) Launch(launchCmd *ExecCommand) (*ProcessState, error) {
 	return nil, fmt.Errorf("operation not supported for legacy exec wrapper")
 }
@@ -133,6 +136,11 @@ func (l *legacyExecutorWrapper) Exec(deadline time.Time, cmd string, args []stri
 	return l.client.Exec(deadline, cmd, args)
 }
 
+func (l *legacyExecutorWrapper) ExecStreaming(ctx context.Context, cmd []string, tty bool,
+	stream drivers.ExecTaskStream) error {
+	return fmt.Errorf("operation not supported for legacy exec wrapper")
+}
+
 type pre09ExecutorRPC struct {
 	client *rpc.Client
 	logger hclog.Logger