add nightly CI runs

[mikemccracken]

we are seeing intermittent atomfs related failures in the stacker test suite (see eg project-stacker/stacker#744) so let's see if they also happen here. The failures are in the mount + umount + mount a tree of images works tests, in the common code between squash and erofs - code from this repo. There's an equivalent test in this repo, so let's exercise it here to narrow things down

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 40.27%. Comparing base (8fde50f) to head (41a59ed).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
+ Coverage   38.59%   40.27%   +1.68%     
==========================================
  Files          24       24              
  Lines        2257     1758     -499     
==========================================
- Hits          871      708     -163     
+ Misses       1208      872     -336     
  Partials      178      178              

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