Make CodeSniffer installer OS independend in case of directory separator

In the PHP_CodeSniffer project for TYPO3 we got an error that the TYPO3SniffPool is not installable on windows.

Here is the ticket: https://forge.typo3.org/issues/58190

It seems to be an error because of the hardcoded directory separator "/".
This PR uses the PHP Constants DIRECTORY_SEPARATOR to be OS independent.

ATTENTION!
This PR is not tested on Windows.