CI changes

[alfredclwong]

1. Try updating ubuntu-18.04 -> ubuntu-latest. This was causing issues a few months back but hopefully they've been worked out now.
2. Move nightly tests to a weekly cron job. This leaves only the core tests as requirements for the CI passing but still keeps us sufficiently up to date with upcoming features.

[codecov-commenter]

Codecov Report

Merging #251 (93616a3) into main (2a12651) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #251   +/-   ##
=======================================
  Coverage   54.68%   54.68%           
=======================================
  Files          69       69           
  Lines        6494     6494           
=======================================
  Hits         3551     3551           
  Misses       2943     2943           

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