Fix installer and downgrader under Windows

[fritzmg]

Fixes the

rename(public\contao-manager.phar.php.install,public\contao-manager.phar.php): Access is denied (code: 5)
in public\contao-manager.phar.php on line 23

error under Windows.

[ausi]

I’d prefer to only do that if the orignal rename($tempFile, $fileName) fails because with these changes the install process is no longer “atomic”.

[aschempp]

Looks good! Could you apply the same to the downgrade.php?

[aschempp]

Thank you @fritzmg