add cron job to nightly tests

[IanMatthewHuff]

For #

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Has a news entry file (remember to thank yourself!)
• Appropriate comments and documentation strings in the code
• Has sufficient logging.
• Has telemetry for enhancements.
• Unit tests & system/integration tests are added/updated
• Test plan is updated as appropriate
• package-lock.json has been regenerated by running npm install (if dependencies have changed)
• The wiki is updated with any design decisions/details.

[IanMatthewHuff]

Even after adding it back in the UI AzDO still seems to be ignoring the UI added schedule for the Nightly build. Some docs seem to imply this is expected, which others seem to imply that while cron is encouraged the UI schedules should still work.

https://docs.microsoft.com/en-us/azure/devops/release-notes/2019/sprint-153-update#use-cron-syntax-to-specify-schedules-in-a-yaml-file

Going forward you won't be able to add or update schedules that have been created using the UI editor. We will continue to honor those schedules for some time, but we recommend that you remove them from the UI and add them to the YAML file.

https://developercommunity.visualstudio.com/content/problem/608227/unable-to-schedule-builds-in-azure-devops-ui.html

As per new experience, we will give preference to traditional schedules in editor if there are any and not use cron schedules at that time. If you delete all schedules from editor, the next update to yaml file will start using cron schedules in it. User can switch to traditional schedules anytime by just adding them back in editor.

[IanMatthewHuff]

Kinda new to the syntax here, this is what I used to check it.
https://crontab.guru/#0_0_*_*_1-5

[codecov-io]

Codecov Report

Merging #7128 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7128      +/-   ##
==========================================
- Coverage   58.46%   58.43%   -0.04%     
==========================================
  Files         485      485              
  Lines       21437    21437              
  Branches     3462     3462              
==========================================
- Hits        12533    12526       -7     
- Misses       8123     8135      +12     
+ Partials      781      776       -5

Impacted Files	Coverage Δ
src/client/common/logger.ts	68.55% <0%> (-4.41%)	⬇️

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