Fix 21569: Set RES path relative to build.d's directory

[MoonlightSentinel]

This ensures the resulting path is correct even for the sources distributed alongside the official releases.

The issues is a regression introduced by #11269.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21569	normal	Unable to rebuild DMD build using win32.mak - v2.095.0

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "stable + dmd#12143"

[PetarKirov]

This ensures the resulting path is correct even for the sources distributed alongside the official releases.

For future reference:

The contents of e.g. http://downloads.dlang.org/releases/2.x/2.095.0/dmd.2.095.0.linux.tar.xz look like this:

dmd2
├── html/
│   └── ...
├── linux/
│   ├── bin32/
│   │   └── ...
│   ├── bin64/
│   │   └── ...
│   ├── lib32/
│   │   └── ...
│   └── lib64/
│   │   └── ...
├── man/
│   └── ...
├── samples/
│   └── ...
├── src/
│   ├── dmd/
│   │   ├── dmd/
│   │   ├── tests/
│   │   ├── vcbuild/
│   │   ├── bootstrap.sh*
│   │   ├── build.d*         <---- the parent dir of `build.d` is not `src/` but `dmd/`
│   │   ├── .dscanner.ini
│   │   ├── osmodel.mak
│   │   ├── posix.mak
│   │   ├── project.ddoc
│   │   ├── README.md
│   │   ├── win32.mak
│   │   └── win64.mak
│   ├── druntime/
│   │   └── ...
│   ├── phobos/
│   │   └── ...
│   └── VERSION
├── license.txt
└── README.TXT