Fix references to cron timestamp to reflect array returned in audit.

[fubarhouse]

This PR fixes the references to the return variable, by referencing $last['system.cron_last'] instead of $last, because $last is a keyed array which otherwise produces the following error:

Warning: date() expects parameter 2 to be integer, array given in /Users/karl/Downloads/drutiny-nightly-rework_drutiny/drutiny-nightly/x/vendor/drutiny/plugin-drupal-8/src/Audit/CronLast.php on line 34

Fatal error: Uncaught Error: Unsupported operand types in /Users/karl/Downloads/drutiny-nightly-rework_drutiny/drutiny-nightly/x/vendor/drutiny/plugin-drupal-8/src/Audit/CronLast.php:36
Stack trace:
#0 /Users/karl/Downloads/drutiny-nightly-rework_drutiny/drutiny-nightly/x/vendor/drutiny/drutiny/src/Audit.php(76): Drutiny\Plugin\Drupal8\Audit\CronLast->audit(Object(Drutiny\Sandbox\Sandbox))
#1 /Users/karl/Downloads/drutiny-nightly-rework_drutiny/drutiny-nightly/x/vendor/drutiny/drutiny/src/Sandbox/Sandbox.php(94): Drutiny\Audit->execute(Object(Drutiny\Sandbox\Sandbox))
#2 /Users/karl/Downloads/drutiny-nightly-rework_drutiny/drutiny-nightly/x/vendor/drutiny/drutiny/src/Assessment.php(58): Drutiny\Sandbox\Sandbox->run()
#3 /Users/karl/Downloads/drutiny-nightly-rework_drutiny/drutiny-nightly/x/vendor/drutiny/drutiny/src/Command/ProfileRunCommand.php(258): Drutiny\Assessment->assessTarget(Object(Drutiny\Target\DrushTarget), Array, Object(DateTime), Object(DateTime), false)

[fiasco]

Hey @fubarhouse which Drush version does this apply to? I presume 9? Because I suspect this change would break with Drush 8 and versions earlier than Drupal 8.3

[fubarhouse]

@fiasco,

This affects Drush version 9.5.2.