Enhance install tool core update action
At the moment you get an error message in the first_install environment step if you don't use a symbolic link for typo3_src. If you use a composer install, or local installations etc. this should never be an error. the same error occurs in the install tool under "folder structure" (in the error message there should be symlink instead of link).
Also the action 'core update' should be disabled in these cases.