chore(ci): Add pre-release workflow

[effigies]

#3671 dropped pre-release tests from branch/PR builds. This restores them as a weekly cron job that generates an issue on failure.

It also runs on any branch called ci/prerelease.

Based on work by @Remi-Gau and me in nipy/nibabel#1466.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.22%. Comparing base (18f3917) to head (d2f6e07).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3677   +/-   ##
=======================================
  Coverage   74.22%   74.22%           
=======================================
  Files          62       62           
  Lines        4989     4989           
  Branches      536      536           
=======================================
  Hits         3703     3703           
  Misses       1154     1154           
  Partials      132      132           

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