Makefile: Slightly simplify recursive make invocations

[kinke]

No need to be explicit about the Makefile name.

[dlang-bot]

Thanks for your pull request and interest in making D better, @kinke! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Testing this PR locally

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dub run digger -- build "master + phobos#8868"