Maint: Disable build isolation in pip cron tests

[Dreamstick9]

Fixes #5125

Changes made in this Pull Request:

• Updated .github/workflows/gh-ci-cron.yaml to include the --no-build-isolation flag in pip install commands.
• This ensures that the nightly cron tests run against the actual environment dependencies (like nightly NumPy builds) rather than installing isolated stable versions.

PR Checklist

• Issue raised/referenced?
• Tests updated/added? (N/A - CI workflow update)
• Documentation updated/added? (N/A)
• package/CHANGELOG file updated? (N/A - Maintenance fix)
• Is your name in package/AUTHORS? (Note: My name is added to AUTHORS in my pending PR MAINT: Replace deprecated numpy assertions in coordinates/base.py #5156)

Developers Certificate of Origin

I certify that I can submit this code contribution as described in the Developer Certificate of Origin, under the MDAnalysis LICENSE.

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📚 Documentation preview 📚: https://mdanalysis--5181.org.readthedocs.build/en/5181/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.72%. Comparing base (9fb8b0a) to head (a117f75).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5181      +/-   ##
===========================================
- Coverage    92.72%   92.72%   -0.01%     
===========================================
  Files          180      180              
  Lines        22472    22472              
  Branches      3188     3188              
===========================================
- Hits         20838    20837       -1     
- Misses        1176     1177       +1     
  Partials       458      458              

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[IAlibay]

Thanks for doing this @Dreamstick9 but there's quite a lot to check here rather than just adding the flags back in, I'm unfortunately going to say this is something one of the core developers should be doing rather than going back and forth doing the checks.

[Dreamstick9]

Thanks @IAlibay! totally understand—CI/build isolation can get tricky with side effects, so it makes sense for a core dev to handle the deep verification.

I'll close this PR so you folks can tackle it directly. Thanks for the quick feedback!