replace old mustc links with new instructions

[brianyan918]

The mustc download links were changed. I'm updating the scripts accordingly.

[codecov]

Codecov Report

Merging #5516 (8f58f35) into master (a4e69bb) will increase coverage by 4.43%.
Report is 5 commits behind head on master.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5516      +/-   ##
==========================================
+ Coverage   70.92%   75.35%   +4.43%     
==========================================
  Files         709      709              
  Lines       65361    65358       -3     
==========================================
+ Hits        46355    49249    +2894     
+ Misses      19006    16109    -2897     

Flag	Coverage Δ
test_configuration_espnet2	∅ <ø> (∅)
test_integration_espnet1	65.69% <ø> (+0.02%)	⬆️
test_integration_espnet2	48.68% <ø> (?)
test_python_espnet1	19.07% <ø> (-0.08%)	⬇️
test_python_espnet2	51.44% <ø> (-0.04%)	⬇️
test_utils	23.10% <ø> (ø)

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see 89 files with indirect coverage changes

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