index: fix chaining order

[pjonsson]

Put the on_conflict_do_update
before the returning.

---

📚 Documentation preview 📚: https://datacube-explorer--875.org.readthedocs.build/en/875/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.32%. Comparing base (f6ddc14) to head (1d2a029).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #875   +/-   ##
========================================
  Coverage    84.32%   84.32%           
========================================
  Files           35       35           
  Lines         4230     4230           
  Branches       537      537           
========================================
  Hits          3567     3567           
  Misses         467      467           
  Partials       196      196           

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