"Portable" `openssl.cafile` location fails to be detected properly on Windows

If the `openssl.cafile` location is used on Windows with a portable path like `openssl.cafile = "\dev\xampp\apache\bin\curl-ca-bundle.crt"`, Composer will fail to properly detect and validate it, falling back to the bundled CA instead.

Note the backslashes and the lack of a drive letter (i.e. `d:`).

Such a setup is used by the portable version of XAMPP on Windows. It checks the folder on the currently active drive for the process.

If the configuration is changed to `openssl.cafile = "d:\dev\xampp\apache\bin\curl-ca-bundle.crt"` it seems to work correctly.

I'm not 100% sure this can or should be solved by Composer, but wanted to keep track of the issue here nevertheless.

See related issue where this was detected: https://github.com/wp-cli/package-command/issues/104#issuecomment-471201646