GHA: Pin Cygwin mingw64-binutils version to 2.37
GHA: Pin Cygwin mingw64-binutils version to 2.37

Cygwin mingw64-binutils upgrade from 2.37 to 2.38 broke the GUI binaries
in Windows. Binaries are produced without error but they don't start.

This hack to pin the binutils version to 2.37 is a workaround that
allows the CI to produce working GUI binaries for Windows.

paulp/unison