Replace phpdbg with php in coverage target

[ohmyfelix]

What changed

• replaced -p phpdbg with -p php in the coverage Makefile target
• kept the change scoped to the single coverage command

Why

• phpdbg is no longer required for this coverage invocation, and using plain php keeps the target aligned with the requested maintenance update
• relates to Remove phpdbg from coverage targets and use php instead contributte#73