CI: cancel old PR jobs

[pjonsson]

Cancel the previous jobs
when a PR is updated.
Merge jobs are never
cancelled.

---

📚 Documentation preview 📚: https://datacube-explorer--657.org.readthedocs.build/en/657/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.27%. Comparing base (6f6d34f) to head (cb235ec).
Report is 41 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #657      +/-   ##
===========================================
+ Coverage    89.26%   89.27%   +0.01%     
===========================================
  Files           34       34              
  Lines         4118     4122       +4     
===========================================
+ Hits          3676     3680       +4     
  Misses         442      442              

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[omad]

Looks good to me, thanks.