Use php in coverage target

[ohmyfelix]

Summary

• replace -p phpdbg with -p php in the coverage target of Makefile
• align this repository with the org-wide coverage migration tracked in #73

Motivation

• phpdbg is being removed from Contributte coverage targets, so this repo should use the regular PHP binary for coverage runs instead

Changes

• update both coverage target variants in Makefile to invoke Tester with -p php

Testing

• make coverage attempted locally
• make tests attempted locally
• CI passes

Local blockers:

• make coverage fails because this environment does not have Xdebug, PCOV, or PHPDBG available for coverage collection when using php
• make tests fails in tests/Cases/File.phpt because the GD extension is not loaded in this environment

Closes #73

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (master@9dfc930). Learn more about missing BASE report.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #45   +/-   ##
=========================================
  Coverage          ?   87.61%           
=========================================
  Files             ?       26           
  Lines             ?      444           
  Branches          ?        0           
=========================================
  Hits              ?      389           
  Misses            ?       55           
  Partials          ?        0           

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