drivers/exec: pass capabilities through executor RPC

Add capabilities to the LaunchRequest proto so that the
capabilities set actually gets plumbed all the way through
to task launch.

drivers/java/driver.go

@@ -491,6 +491,7 @@ func (d *Driver) StartTask(cfg *drivers.TaskConfig) (*drivers.TaskHandle, *drive
 	if err != nil {
 		return nil, nil, err
 	}
+	d.logger.Debug("task capabilities", "capabilities", caps)
 
 	execCmd := &executor.ExecCommand{
 		Cmd:              absPath,