mod_cron: Fix recording last task run time #1751

The type checks, they do nothing!

Observed: Tasks that were supposed to run weekly or daily were running
each hour.

plugins/mod_cron.lua

@@ -45,6 +45,7 @@ local function should_run(when, last) return not last or last + periods[when] *
 local function run_task(task)
 	local started_at = os.time();
 	task:run(started_at);
+	task.last = started_at;
 	task:save(started_at);
 end
 

teal-src/plugins/mod_cron.tl

@@ -84,6 +84,7 @@ end
 local function run_task(task : task_spec)
 	local started_at = os.time();
 	task:run(started_at);
+	task.last = started_at;
 	task:save(started_at);
 end