fix bug when feats.scp changes name

[Baileyswu]

Hi, mravanelli! When I change the feature type (mfcc to fbank), the scp file changes (feats.scp to feats_fbank.scp). However, during the decoding process at decode_dnn.sh, the filename is frozen as feats.scp, which causes trouble. Also, the files_dec at run_exp.py needs to be specific in my env.