Legacy executors are executors after all

[notnoop]

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

nomad/drivers/shared/executor/utils.go

Lines 103 to 110 in b312aac

	raw, err := rpcClient.Dispense("executor")
	if err != nil {
	return nil, nil, fmt.Errorf("unable to dispense the executor plugin: %v", err)
	}
	executorPlugin, ok := raw.(Executor)
	if !ok {
	return nil, nil, fmt.Errorf("unexpected executor rpc type: %T", raw)
	}

.

Impact

The bug affects all versions since 0.9.2: if a client upgrades from pre-0.9 to 0.9.2 or later; then all exec based tasks (i.e. exec, raw_exec, java, qemu) will fail to recover, will be leaked, and then nomad will start another task. The leaked pre-0.9 task will run un-interrupted and unmanaged until the client dies or it's killed manually.

[schmichael]

Oof great catch.

[github-actions]

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