shell_exec: please check the PHP executable we are running on

We have an environment with different PHP versions and `php` is linked to our latest, probably not suitable for the Magento version we are working on or with other ini-settings applied.

We just start the Composer `bin/magento`  script with a custom PHP executable, eg. `php7.4`

So instead of `php` as executable use the current executable which was applied by the user.