[BUGFIX] Use correct absolute namespace for Environment::class

When using the extension with TYPO3 10, I ran into this error message:

PHP Warning: Use of undefined constant PATH_site - assumed 'PATH_site' (this will throw an Error in a future version of PHP) in /var/www/private/typo3conf/ext/deployer_information/Classes/ToolbarItem/LastDeployInformation.php line 44

The problem was an incomplete/relative class namespace TYPO3\CMS\Core\Core\Environment which was resolved as SpoonerWeb\DeployerInformation\ToolbarItem\TYPO3\CMS\Core\Core\Environment.

To solve this, I added a leading \ to make the namespace absolute. Now the class can be found and the extension works properly with my TYPO3 10 installation.