Add installers/install_ffmpeg.sh

[kamo-naoyuki]

I created installers/install_ffmpeg.sh

• If you have conda -> Install ffmpeg using conda
• If you don't have conda -> Install ffmpeg using installers/install_ffmpeg.sh

[codecov]

Codecov Report

Merging #5046 (9fb3623) into master (0b24151) will decrease coverage by 2.52%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5046      +/-   ##
==========================================
- Coverage   55.85%   53.33%   -2.52%     
==========================================
  Files         494      483      -11     
  Lines       43791    42732    -1059     
==========================================
- Hits        24460    22792    -1668     
- Misses      19331    19940     +609     

Flag	Coverage Δ
test_integration_espnet1	66.28% <ø> (ø)
test_integration_espnet2	43.94% <ø> (-4.03%)	⬇️
test_utils	?

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