Fix build path on Windows

[ericphanson]

as noticed in https://discourse.julialang.org/t/baron-error-ioerror-could-not-spawn/44430, the path that gets written only has single / instead of // on Windows.

I tested this new way locally on Windows and WSL via

path = pwd()
open("file.txt", "w") do io
    write(io, """const baron_exec = $(repr(path))\n""")
end

and it escapes things correctly in each case (const baron_exec = "C:\\Users\\eric\\Code" on Windows and const baron_exec = "/mnt/c/Users/eric" in WSL).

[joehuchette]

Thanks for this. I don't have a Windows machine for testing, so I'll take your word for it :)