Windows: ghc-install.sh does not output absolute path to a GHC binary

[mpilgrem]

Stack's GHC installation customisation requries that the script output the absolute path to the GHC binary. On Windows, an executable will have file extension .exe. However, ghc-install.sh has this:

printf "%s/ghc" "${ghcdir}"

On Windows, I think that should be:

printf "%s/ghc.exe" "${ghcdir}"

I think this causes Stack to report warnings like this one, because Stack checks that the compiler executable file exists:

Warning: Unable to save CompilerPaths cache:

         CreateFile "D:\\ghcup\\ghc\\9.0.2\\bin\\ghc": does not exist (The system cannot find the file specified.)

See: commercialhaskell/stack#6330

[mpilgrem]

So, perhaps:

if [ -n "$WINDIR" ]; then
    # Running on Windows
    printf "%s/ghc.exe" "${ghcdir}"
else
    # Not running on Windows
    printf "%s/ghc" "${ghcdir}"
fi