CI: Adapt to generic Makefile for Phobos

[kinke]

Depends on dlang/phobos#8862 (vice-versa).

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Testing this PR locally

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[kinke]

This is a slipstreamed fix for dmd -g -debuglib= foo.d causing an invalid /DEFAULTLIB: linker flag for link.exe, which complains about a missing name. The same logic is used for -defaultlib= a few lines up.

[kinke]

(The Phobos Makefile specifies -defaultlib= -debuglib= to make sure no default Phobos library is linked.)

[ibuclaw]

Why was this merged without waiting for all pipelines to succeed?

[thewilsonator]

#15920 (comment)

Because of a mutual two way dependance on dlang/phobos#8862 which you merged

[ibuclaw]

#15920 (comment)

Because of a mutual two way dependance on dlang/phobos#8862 which you merged

And then I retriggered CI here to confirm that was the case.