Add ferretdb_ prefix to our custom build tags

[AlekSi]

To avoid possible conflicts.

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, Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #869 (cce19a7) into main (d687477) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #869      +/-   ##
==========================================
- Coverage   59.37%   59.34%   -0.04%     
==========================================
  Files         218      218              
  Lines        9833     9833              
==========================================
- Hits         5838     5835       -3     
- Misses       3299     3301       +2     
- Partials      696      697       +1     

Impacted Files	Coverage Δ
internal/util/version/version.go	55.00% <0.00%> (-5.00%)	⬇️

Flag	Coverage Δ
integration	61.83% <0.00%> (-0.04%)	⬇️
mongodb	18.92% <0.00%> (-0.04%)	⬇️
pg	61.76% <0.00%> (-0.04%)	⬇️
unit	24.93% <0.00%> (-0.04%)	⬇️

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

[rumyantseva]

LGTM