Use php in coverage target

[ohmyfelix]

Summary

• replace phpdbg with php in the coverage target of Makefile
• keep both GitHub Actions and local coverage commands aligned

Motivation

• contributte/contributte#73 tracks removing phpdbg from Contributte coverage targets

Changes

• update the CI coverage command to use -p php
• update the local HTML coverage command to use -p php

Testing

• Verified make -n coverage prints the php command
• Full coverage run executed locally
• CI passes

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