refact cmd/crowdsec: encapsulate cache into alertBuffer

[mmetc]

simplify loop and reduce risk of races (we didn't lock before reading len)

[github-actions]

@mmetc: There are no area labels on this PR. You can add as many areas as you see fit.

• /area agent
• /area local-api
• /area cscli
• /area appsec
• /area security
• /area configuration

Details

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[codecov]

Codecov Report

❌ Patch coverage is 62.06897% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.10%. Comparing base (1df2c55) to head (e01dcd1).

Files with missing lines	Patch %	Lines
cmd/crowdsec/output.go	62.06%	9 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4300      +/-   ##
==========================================
- Coverage   63.11%   63.10%   -0.01%     
==========================================
  Files         473      473              
  Lines       33466    33468       +2     
==========================================
- Hits        21123    21121       -2     
- Misses      10222    10226       +4     
  Partials     2121     2121              

Flag	Coverage Δ
bats	46.55% <62.06%> (-0.02%)	⬇️
unit-linux	36.00% <0.00%> (-0.07%)	⬇️
unit-windows	24.45% <0.00%> (+0.01%)	⬆️

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