Use the coverages that are persisted within the action if possible

[uhafner]

Otherwise, the whole coverage result XML will be loaded just for a single value.

Will fix jenkins-infra/helpdesk#3029.

[codecov]

Codecov Report

Merging #423 (de934ea) into master (9241b76) will increase coverage by 0.09%.
The diff coverage is 81.25%.

@@             Coverage Diff              @@
##             master     #423      +/-   ##
============================================
+ Coverage     72.08%   72.18%   +0.09%     
- Complexity      984      992       +8     
============================================
  Files            86       86              
  Lines          3694     3700       +6     
  Branches        431      433       +2     
============================================
+ Hits           2663     2671       +8     
+ Misses          885      883       -2     
  Partials        146      146              

Impacted Files	Coverage Δ
...ns/plugins/coverage/model/CoverageBuildAction.java	78.72% <80.00%> (+3.72%)	⬆️
...nkins/plugins/coverage/model/CoverageReporter.java	81.60% <100.00%> (ø)

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[medianick]

Thanks @uhafner! I look forward to trying this fix. As I noted on the linked issue, our PR pages for multibranch pipeline jobs started taking over a minute to render a list of 100 PRs. We hadn't yet tracked down the cause but it coincided with the initial upgrade to 3.0.0.