Replace phpdbg with php in coverage target

[ohmyfelix]

Summary

• replace -p phpdbg with -p php in the coverage target for both CI and local runs

Motivation

This keeps contributte/messenger aligned with the org-wide coverage migration tracked in contributte/contributte#73.

Changes

• update the CI coverage command in Makefile
• update the local coverage command in Makefile

Testing

• make coverage locally (php coverage is blocked here because this environment has no Xdebug or PCOV extension installed)
• make tests locally (fails in this environment on PHP 8.5.3 because DI/MessengerExtension.handler.phpt errors with "Cannot use object of type Generator as array" and DI/MessengerExtension.transport.phpt requires php-redis 4.3.0 or higher)
• CI passes

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