[llvm] [flang-rt] Use correct flang-rt build for flang-rt unit tests on Windows (PR #152318)

[via llvm-commits]

llvmbot wrote:

Failed to create pull request for issue152318 https://github.com/llvm/llvm-project/actions/runs/16804051954

https://github.com/llvm/llvm-project/pull/152318