PHP version pinned for WP-CLI

[julesfell]

A problem has been encountered here:

https://github.com/BeAPI/wp-cli-site-duplicator-command/blob/master/classes/command.php#L289-L301

Using WP_CLI::runcommand( $_command ) does not allow you to choose which version of PHP to use for running WP-CLI.

Sometimes the command executed generates an error due to an incorrect version of PHP set by default for wp <command>. It could be useful to be able to decide which version of PHP to use, in order to ensure that the WP-CLI utility runs.