build: use $(BUILDTYPE) when cleaning coverage files

[avivkeller]

These checks would previously fail in the case that a user built a Debug-only build of the project, now, it checks $(BUILDTYPE), so any build type passes the check.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.74%. Comparing base (7bc37af) to head (e45ada0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #57995      +/-   ##
==========================================
- Coverage   90.26%   89.74%   -0.53%     
==========================================
  Files         630      630              
  Lines      186168   186168              
  Branches    36475    36165     -310     
==========================================
- Hits       168053   167071     -982     
- Misses      10977    11935     +958     
- Partials     7138     7162      +24     

see 92 files with indirect coverage changes

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