Makefile: use php in coverage target

[ohmyfelix]

Summary

Switch the coverage target in Makefile from phpdbg to php as part of the org-wide coverage migration.

Motivation

phpdbg is being removed from Contributte coverage targets in contributte/contributte#73 so repositories use the standard PHP binary consistently.

Changes

• replace -p phpdbg with -p php in the CI coverage command
• replace -p phpdbg with -p php in the local coverage command

Testing

• Tests pass locally (make tests)
• Coverage passes locally (make coverage fails because this environment does not have Xdebug or PCOV enabled for php)
• CI passes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (master@b4d408a). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff            @@
##             master     #14   +/-   ##
========================================
  Coverage          ?   2.35%           
========================================
  Files             ?       2           
  Lines             ?      85           
  Branches          ?       0           
========================================
  Hits              ?       2           
  Misses            ?      83           
  Partials          ?       0           

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