fix a data preparation issue for librimix recipe.

[LiChenda]

In egs2/librimix/asr1/local/data.sh, L116, if mix_f matches multiple files, the grep command (in some version) will output the specific matched file name (which is not the expected output) and lead to a wrong utt_id in data preparation.

So I updated the data preparation scripts to avoid this issue.

[sw005320]

Thanks, @LiChenda!

[codecov]

Codecov Report

Merging #5322 (394a27e) into master (17b472d) will increase coverage by 3.58%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5322      +/-   ##
==========================================
+ Coverage   72.52%   76.10%   +3.58%     
==========================================
  Files         658      658              
  Lines       59156    59156              
==========================================
+ Hits        42902    45022    +2120     
+ Misses      16254    14134    -2120     

Flag	Coverage Δ
test_integration_espnet1	65.96% <ø> (ø)
test_integration_espnet2	47.52% <ø> (?)
test_python	66.49% <ø> (ø)
test_utils	23.17% <ø> (ø)

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