Makefile: replace phpdbg with php in coverage target

[ohmyfelix]

What changed

• replaced -p phpdbg with -p php in both coverage target variants in Makefile
• kept the rest of the coverage command unchanged

Why

• aligns this repository with the org-wide phpdbg removal tracked in Remove phpdbg from coverage targets and use php instead contributte#73

Testing

• verified the expanded command with make -n coverage

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