Dataset directory naming causes "File name too long" error

[light1726]

Thanks for your wonderful repo!

Just found this very little bug: Here if one specifies several dozens of speakers to be used for training, https://github.com/kan-bayashi/ParallelWaveGAN/blob/b9e740c67f77ff689abf32edbeff825bc7f7582e/egs/vctk/voc1/run.sh#L20

then the following line will produce a super long directory name that cannot be created with "mkdir" later. https://github.com/kan-bayashi/ParallelWaveGAN/blob/b9e740c67f77ff689abf32edbeff825bc7f7582e/egs/vctk/voc1/run.sh#L39