I have no idea how bors keeps working without this - I can only assume it's some peculiarity of how windows searches for DLLs.
Without this change, running `make check` on windows will not correctly set PATH to include eg. `x86_64-pc-windows-gnu\stage1\bin\rustlib\x86_64-pc-windows-gnu\lib`, and when it tries to run eg. `stage1/test/stdtest-x86_64-pc-windows-gnu.exe`, it will fail because windows can't find the DLLs on which it relies.
It seems to be just a mistake: when the equivalent was added for the branch that deals with unix-like platforms, the windows branch was left unchanged.
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