Split out stable CI

[icweaver]

Also swap nightly for pre

Another test example for JuliaAstro/JuliaAstro.github.io#158

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.70%. Comparing base (d34e969) to head (93027a9).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #85   +/-   ##
=======================================
  Coverage   94.70%   94.70%           
=======================================
  Files          11       11           
  Lines         737      737           
=======================================
  Hits          698      698           
  Misses         39       39           

Flag	Coverage Δ
unittests	94.70% <ø> (ø)

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