Use CODECOV_TOKEN if available

[AlekSi]

Description

That should make codecov uploads more stable.

Readiness checklist

• I added tests for new functionality or bugfixes.
• I ran task all, and it passed.
• I added/updated comments for both exported and unexported top-level declarations (functions, types, etc).
• I checked comments rendering with task godocs.
• (for maintainers only) I set Reviewers (@FerretDB/core), Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #1073 (02fe29d) into main (3f13625) will increase coverage by 0.06%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1073      +/-   ##
==========================================
+ Coverage   64.70%   64.76%   +0.06%     
==========================================
  Files         246      246              
  Lines       11527    11527              
==========================================
+ Hits         7459     7466       +7     
+ Misses       3204     3197       -7     
  Partials      864      864              

Impacted Files	Coverage Δ
internal/util/version/version.go	55.00% <0.00%> (-5.00%)	⬇️
internal/clientconn/listener.go	76.04% <0.00%> (-3.13%)	⬇️
internal/wire/msg_header.go	69.04% <0.00%> (-2.39%)	⬇️
internal/wire/msg_body.go	36.50% <0.00%> (-1.59%)	⬇️
internal/handlers/common/filter.go	86.36% <0.00%> (+0.97%)	⬆️
internal/handlers/pg/pgdb/settings.go	44.18% <0.00%> (+4.65%)	⬆️

Flag	Coverage Δ
integration	60.62% <ø> (-0.09%)	⬇️
mongodb	18.51% <ø> (-0.02%)	⬇️
pg	62.80% <ø> (-0.11%)	⬇️
tigris	29.39% <ø> (-0.03%)	⬇️
unit	25.32% <ø> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

[rumyantseva]

LGTM