CI: Rename Windows installer artifacts to dmd-<branch>.exe

[kinke]

Don't use the DMD VERSION file, currently leading to names like dmd-v2.110.0-beta.1.exe, which accumulate for the nightly release: https://github.com/dlang/dmd/releases/tag/nightly

Switch to dmd-master.exe, similar to the other artifact names.

[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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