fix: Add distinct to query that is causing high CPU on retention jobs

[bbytheway-rq]

fixes #22319

Adds a distinct keyword to ensure the query get only the unique digests, not repeated rows for every time the blob is used by multiple artifacts.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 65.84%. Comparing base (c8c11b4) to head (ca94c4d). :warning: Report is 606 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #22634       +/-   ##
===========================================
+ Coverage   45.36%   65.84%   +20.48%     
===========================================
  Files         244     1073      +829     
  Lines       13333   116095   +102762     
  Branches     2719     2931      +212     
===========================================
+ Hits         6049    76448    +70399     
- Misses       6983    35408    +28425     
- Partials      301     4239     +3938     

Flag	Coverage Δ
unittests	65.84% <100.00%> (+20.48%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/pkg/blob/dao/dao.go	58.13% <100.00%> (ø)

... and 986 files with indirect coverage changes

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