fix format_wav_scp.py

[kamo-naoyuki]

The processing for cache has a bug.

[codecov]

Codecov Report

Merging #5150 (3a37786) into master (866c801) will increase coverage by 0.00%.
The diff coverage is 70.00%.

@@           Coverage Diff            @@
##           master    #5150    +/-   ##
========================================
  Coverage   74.98%   74.99%            
========================================
  Files         617      618     +1     
  Lines       55176    55585   +409     
========================================
+ Hits        41376    41686   +310     
- Misses      13800    13899    +99     

Flag	Coverage Δ
test_integration_espnet1	66.28% <ø> (-0.01%)	⬇️
test_integration_espnet2	47.61% <70.00%> (-0.15%)	⬇️
test_python	65.45% <70.00%> (+0.07%)	⬆️
test_utils	23.28% <ø> (ø)

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Impacted Files	Coverage Δ
espnet2/fileio/sound_scp.py	97.50% <70.00%> (-1.63%)	⬇️

... and 21 files with indirect coverage changes

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