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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