Clean env for user specific cronjob

• user specific cronjob env should be reset when schedule

related: webdevops#39

ref: https://github.com/cronie-crond/cronie/blob/630b24112fbd87c0dd020795a221fefc8840630a/src/security.c#L261
ref: https://github.com/cronie-crond/cronie/blob/630b24112fbd87c0dd020795a221fefc8840630a/src/do_command.c#L337
ref: https://github.com/cronie-crond/cronie/blob/630b24112fbd87c0dd020795a221fefc8840630a/src/env.c#L123
ref: https://github.com/cronie-crond/cronie/blob/630b24112fbd87c0dd020795a221fefc8840630a/src/env.c#L291
ref: crontab 手册

Init metrics when scheduler started

douban-v23.4.1

Add task run next/prev time metrics

Add extra metrics

# HELP gocrond_task_run_next_time gocrond task next run ts
# TYPE gocrond_task_run_next_time gauge
gocrond_task_run_next_time{cronCommand="test.sh",cronSpec="*/3 * * * *",cronUser="test"} 1.68198966e+09
# HELP gocrond_task_run_prev_time gocrond task prev run ts
# TYPE gocrond_task_run_prev_time gauge
gocrond_task_run_prev_time{cronCommand="test.sh",cronSpec="*/10 * * * *",cronUser="test"} 1.6819896e+09