Fix unsafe math rand usage

[xiaokangwang]

This pull request replaced unsafe math rand usage with crypto/rand for passively observable random choices.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 60.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 35.65%. Comparing base (cc79bbe) to head (ee604e3).

Files	Patch %	Lines
common/dice/dice.go	66.66%	1 Missing and 1 partial ⚠️
infra/conf/v4/trojan.go	0.00%	1 Missing ⚠️
proxy/shadowsocks2022/encoding.go	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2926      +/-   ##
==========================================
+ Coverage   35.61%   35.65%   +0.03%     
==========================================
  Files         729      729              
  Lines       41550    41556       +6     
==========================================
+ Hits        14799    14817      +18     
+ Misses      25115    25105      -10     
+ Partials     1636     1634       -2     

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