Replace phpdbg with php in coverage target

[ohmyfelix]

What changed

• replaced -p phpdbg with -p php in the coverage target of Makefile
• kept both GitHub Actions and local coverage commands otherwise unchanged

Why

• aligns this repository with the org-wide phpdbg removal tracked in Remove phpdbg from coverage targets and use php instead contributte#73
• keeps coverage runnable with the standard PHP binary

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 34.18%. Comparing base (49ff2e8) to head (6b773d1).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
- Coverage   38.39%   34.18%   -4.21%     
==========================================
  Files           3        3              
  Lines         112      117       +5     
==========================================
- Hits           43       40       -3     
- Misses         69       77       +8     

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