Allow setting "package" variable with a testing flag

[AlekSi]

Description

That's needed for environments like Civo.

Readiness checklist

• I added/updated unit tests.
• I added/updated integration/compatibility tests.
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[AlekSi]

@rumyantseva Please set FERRETDB_TEST_TELEMETRY_PACKAGE: civo in your PR

[rumyantseva]

Looks helpful!

[codecov]

Codecov Report

Merging #2357 (5bb3057) into main (eafe7f5) will decrease coverage by 36.77%.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2357       +/-   ##
===========================================
- Coverage   63.99%   27.22%   -36.77%     
===========================================
  Files         391      391               
  Lines       18993    18987        -6     
===========================================
- Hits        12155     5170     -6985     
- Misses       5931    13262     +7331     
+ Partials      907      555      -352     

Impacted Files	Coverage Δ
build/version/version.go	67.64% <100.00%> (+0.07%)	⬆️

... and 125 files with indirect coverage changes

Flag	Coverage Δ
integration	4.79% <0.00%> (-52.09%)	⬇️
mongodb	4.79% <0.00%> (+<0.01%)	⬆️
pg	?
unit	25.88% <100.00%> (-0.02%)	⬇️

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

[w84thesun]

👍