fix(diagnostic): only set coc_diagnostic_map on displayByVimDiagnostic

[fannheyward]

Closes #5242

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.35%. Comparing base (674482c) to head (9aa7edb).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/diagnostic/buffer.ts	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5245      +/-   ##
==========================================
- Coverage   97.36%   97.35%   -0.02%     
==========================================
  Files         291      291              
  Lines       27038    27040       +2     
  Branches     5610     5611       +1     
==========================================
- Hits        26326    26325       -1     
- Misses        495      498       +3     
  Partials      217      217              

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