Fix Makefile clean target failing to delete build artifact.

[NlightNFotis]

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
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[codecov]

Codecov Report

Merging #5853 (0f9bfa7) into develop (4d7158f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #5853   +/-   ##
========================================
  Coverage    72.87%   72.88%           
========================================
  Files         1421     1421           
  Lines       154208   154262   +54     
========================================
+ Hits        112386   112434   +48     
- Misses       41822    41828    +6     

Impacted Files	Coverage Δ
src/analyses/goto_check.cpp	88.28% <100.00%> (+0.02%)	⬆️
src/util/format_expr.cpp	85.77% <0.00%> (+0.77%)	⬆️

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