Fix coverage target to use php

[ohmyfelix]

Summary

• replace -p phpdbg with -p php in the coverage target
• keep both GitHub Actions and local coverage invocations aligned

Motivation

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

Changes

• update the CI coverage command to run Tester with php
• update the local coverage command to run Tester with php

Testing

• make -n coverage
• Full coverage run

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