Use php for coverage target

[ohmyfelix]

Summary

• replace -p phpdbg with -p php in the coverage Make target
• keep the coverage command aligned with environments where phpdbg is unavailable

Why

• phpdbg is being removed from this workflow, so coverage should run with the standard PHP binary instead
• relates to Remove phpdbg from coverage targets and use php instead contributte#73

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.53%. Comparing base (af0c383) to head (4be7907).
⚠️ Report is 15 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #426      +/-   ##
==========================================
+ Coverage   58.33%   61.53%   +3.20%     
==========================================
  Files           2        2              
  Lines          24       26       +2     
==========================================
+ Hits           14       16       +2     
  Misses         10       10              

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