Use the correct relative-path separator under Windows

[nonk123]

For reasons I cannot comprehend, having ci-run-arguments set to a non-empty string and executing M-x ci-run-last-target (on Windows 10) produces the following error message:

./dwarfpaper.exe -D -m forest
'.' is not recognized as an internal or external command,
operable program or batch file.

Replacing the forward-slash / with a backslash \ solved the issue, at least for my case.

What confuses me the most, is that this issue never occurred until I decided to set ci-run-arguments. Running executables with the forward-slash / separator (./dwarfpaper.exe) but without the args, worked fine all this time I've been using the package.

Let me know if you have any suggestions for this PR.