chore: Prune Ubuntu environment before building docker images

[effigies]

Docker builds are suddenly failing for a lack of disk space. Trying out a suggestion in https://github.com/orgs/community/discussions/179725.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.93%. Comparing base (d88a28f) to head (376f963).
⚠️ Report is 27 commits behind head on maint/25.2.x.

Additional details and impacted files

@@              Coverage Diff              @@
##           maint/25.2.x    #3571   +/-   ##
=============================================
  Coverage         72.92%   72.93%           
=============================================
  Files                60       60           
  Lines              4817     4818    +1     
  Branches            625      625           
=============================================
+ Hits               3513     3514    +1     
  Misses             1162     1162           
  Partials            142      142           

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