README: Add --locked to the cargo install

[sylvestre]

For #1786

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.01 ⚠️

Comparison is base (7416904) 29.44% compared to head (6a96f88) 29.43%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1787      +/-   ##
==========================================
- Coverage   29.44%   29.43%   -0.01%     
==========================================
  Files          49       49              
  Lines       17713    17713              
  Branches     8547     8548       +1     
==========================================
- Hits         5215     5214       -1     
+ Misses       7347     7344       -3     
- Partials     5151     5155       +4     

see 10 files with indirect coverage changes

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