CI: replace phpdbg with php in coverage

[ohmyfelix]

Summary

• run coverage with \ instead of deprecated \
• keep the existing tester coverage target intact otherwise

Motivation

• resolves Remove phpdbg from coverage targets and use php instead contributte#73 for this repository

Testing

• make -n coverage
• full test suite

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (master@b92dfd6). Learn more about missing BASE report.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #134   +/-   ##
=========================================
  Coverage          ?   94.33%           
=========================================
  Files             ?       14           
  Lines             ?      406           
  Branches          ?        0           
=========================================
  Hits              ?      383           
  Misses            ?       23           
  Partials          ?        0           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.